chapter 8. removing specific water contaminant

8.1REMOVING SUSPENDED SOLID CONTAM-INANTS Algae Control Carbon Particles

Gravity Settling Solids Disposal

Foundry Sand Laundry Wastes

The Problem of Commercial Waste Treatment Systems Quality of Effluent

Mill Scale Design Parameters Operational History

Mineral Tailings

8.2REMOVING ORGANIC CONTAMINANTS Aldehydes

Biological Oxidation Air Stripping Carbon Adsorption

Cellulose Pulp Wastewater Volume Effluent Characteristics Methods of Treatment Research Problems

Food Processing Wastes Water Reuse Water Conservation Elimination of Water Use Wastewater Treatment

Hydrocarbons Design Basis

Operational History Pesticides

Pesticide Removal in Natural AquaticSystems

Biodegradable Replacement andControlled Self-Destruction

Biological Treatment Processes Chemical Flocculation and

Oxidation Activated Carbon Adsorption Reverse Osmosis Incineration Research Trends

Phenol Solvent Extraction Biological Treatment Carbon Adsorption Chemical Oxidation

Starch Biological Treatment

Textile Industry Wastes Viruses and Bacteria

Chlorination Ozonation

8.3REMOVING INORGANIC CONTAMINANTS Aluminum Bicarbonate

Removing Bicarbonate Alkalinity Cadmium

Sources of Cadmium-bearing Waste-waters

Treatment Methods Calcium

8Removing Specific WaterContaminantsI.M. Abrams | D.B. Aulenbach | E.C. Bingham | L.J. Bollyky | T.F.Brown, Jr. | B. Bruch | R.D. Buchanan | L.W. Canter | C.A.Caswell | R.A. Conway | G.J. Crits | E.W.J. Diaper | J.W.T. Ferretti| R.G. Gantz | W.C. Gardiner | L.C. Gilde, Jr. | E.G. Kominek |D.H.F. Liu | A.F. McClure, Jr. | F.L. Parker | R.S. Robertson |D.M. Rock | C.J. Santhanam | L.S. Savage | S.E. Smith | F.B.Taylor | C.C. Walden | R.H. Zanitsch

©1999 CRC Press LLC

Chromium Reduction and Precipitation Ion Exchange

Cyanides Chlorination Ozonation

Fluoride Hardness

Ion Exchange Lime and Lime-Soda Ash Softening

Iron Controlling Iron with Bacteria Removing Iron Salts

Lead Treatment Methods

Magnesium Manganese Mercury

Properties Sources of Contamination Methylation of Inorganic Mercury Methods of Removal from Water

Nickel Silica

Insoluble Silica Soluble Silica

Strontium Sulfate

Ion Exchange Evaporation and Crystallization Reverse Osmosis Biological Reduction

Sulfide Zinc

Ion Exchange Precipitation

8.4INORGANIC NEUTRALIZATION ANDRECOVERY Boiler Blowdown Water Spent Caustics from Refineries

Phenolic Sulfidic

Steel Mill Pickle Liquor The Pickling Process Disposition of Spent Liquor

8.5OIL POLLUTION Effects on Plant and Animal Life

Toxicity Marine Organisms Plants and Oil

Sources and Prevention Oily Materials Detection, Identification, and

Surveys Prevention

Methods of Control Characteristics and Composition Mechanical Containment Mechanical Recovery Application of Agents

8.6PURIFICATION OF SALT WATER Conversion Processes Desalination Plants Desalting Processes

Multieffect Evaporation Vapor Compression Evaporation Multiflash Evaporators

Freezing Processes Vacuum-Freeze Vapor Com-

pression Reverse Osmosis Electrodialysis The Future of Desalination

8.7RADIOACTIVE LIQUID WASTETREATMENTLow-Activity Wastes

Precipitation Ion Exchange Evaporators Dilution and Release Hydrofracture Bituminization

High-Activity Wastes Generation Storage in Tanks Conversion to Solids Storage


Algae ControlThe types of algae and the concentration in wastewaterdepend on residence time, climate and weather, amountof pollutants entering the pond, and dimensions of thepond. Normally, small unicellular types of algae developfirst, e.g., Chlorella. Because of their physical dimensionsthey are difficult to remove by the processes listed in Table8.1.1. Longer residence times lead to the development oflarger algae and other plankton, which is more readily re-moved. The algae concentration affects the choice of re-moval process and the rate of treatment. Because of theirlight density, the dried weight of suspended solids is notan efficient measure of concentration. Algae are normallymeasured in volumetric or areal standard units (Anon.1971). In surface water supplies, concentrations may beas high as 30,000 cells per milliliter (ml), this can be muchhigher in nutrient-rich waste treatment effluents. A com-bination of processes may be the best treatment, e.g., cop-per sulfate addition and microstraining, as used on surfacewater supplies in London, England.

Carbon ParticlesCarbon particulate matter suspended in waste effluentmust be either controlled or removed prior to discharge.Wastes associated with the carbon black and acetylene in-dustries are of concern. These wastes may contain up to1000 milligrams per liter (mg/l) carbon particles in sus-pension; in most cases this carbon concentration must be

reduced to less than 50 mg/l suspended solids. Usually,these solids settle readily and are removed by gravity set-tling and/or flotation.

Individual particle sizes range from a submicron tolarger than 100 micron (m). Larger particles settle, whereassmaller particles float. Transition size particles remain sus-pended almost indefinitely unless forced out of suspensionby mechanical or chemical means. Unless a highly clari-fied effluent is required, suspended matter may not haveto be removed as it amounts to a small proportion of to-tal solids concentration.

GRAVITY SETTLING

Two types of gravity systems are available: (1) settlingLagoons, which provide retention time for solid particlesto settle as sludge. These must be cleaned periodically; and(2) mechanical Clarifiers, which remove suspended solidsand also rid bottom sludges mechanically.

The settling lagoon requires a minimum capital invest-ment. Cleanout costs are high compared with the me-chanical clarifier operating costs.

Settling devices are usually designed on the basis of over-flow rate, gal per day (gpd) per sq ft of surface area.According to the Ten State Standards (Great Lakes-UpperMississippi River Board of State Sanitary Engineers 1968),this rate should be in the range of 600 to 1000 gpd/sq ft.In designing the carbon settling lagoon, frequency of la-goon cleaning must be considered, and the lagoon mustbe sized accordingly. Carbon sludge will settle to a den-sity of 5–20% solids.


8.1REMOVING SUSPENDED SOLID CONTAMINANTS

TABLE 8.1.1 ALGAE REMOVAL PROCESSES: MERITS AND FLAWS

AlgaeRemovalProcess Advantages Limitations

Copper sulfate Simple and inex- Creates toxicity; only some algalpensive forms attacked

Chlorine Simple and inex- High doses needed; not all algaepensive attacked

Coagulation and Positive removal of High chemical doses needed; dif-settling all types of algae ficult sludges produced

Sand filters Positive removal of Rapid filter clogging may occurall types of algae

Microstraining Simple and inex- Not all algal forms removedpensive

Air flotation Positive removal of Not all algal forms removed;all types of algae sludges may be difficult to handle

As an example, a 5-acre lagoon, 5 ft deep, with an in-fluent suspended solids concentration of 1000 mg/l and aneffluent concentration of 50 mg/l at a flowrate of 10 mgdwill retain almost 80,000 lb of solids per day. If the solidssettle to a 5% sludge density, the lagoon will be filled withsludge in less than two months, as indicated by the calcu-lations in Table 8.1.2. A settling lagoon design for this ap-plication would probably be based on cleaning frequencyrather than on overflow rates.

The outfall structure of a settling system should retainfloating material and maintain laminar flow to preventsolids from resuspending at discharge due to turbulence.An underflow-overflow weir (Figure 8.1.1) efficiently pro-vides such an outfall. According to the Ten State Standards(Great Lakes–Upper Mississippi 1968), weir loading ratesshould not exceed 10,000 gpd per linear ft of weir to as-sume minimum resuspension of settled matter from tur-bulent flow. For the example in Table 8.1.2, a weir 1000ft long would be required.

SOLIDS DISPOSAL

Whether a mechanical clarifier, a settling lagoon or othermeans of solids removal is utilized, concentrated carbonslurry or sludge must be disposed of. Disposal methods in-clude incineration, landfill disposal, reuse, and dewatering.Removal and disposal of concentrated solids slurry is themost difficult part of the carbon clarification system.

Eliminating waste at the source is ideal. Tightening pro-duction controls and modifying the process can drastically

reduce waste losses and should be investigated before anyremoval system is developed. No treatment system is jus-tifiable without assurance that waste production is mini-mized at the source. Frequently, waste carbon is a prod-uct loss, and recovery is valuable. Keeping carbon out ofwastewater prevents problems in waste treatment.

Foundry SandFoundry melting emissions contain solid particles rangingfrom coarse dust to fines of submicron size. Cupola emis-sions are much coarser than electric furnace emissions,which are generally less than 5 m.

Foundry melting dusts include combustibles containing20–30% carbonaceous material. Iron oxides account fornearly 60% of collected dusts; silica and miscellaneousmetallic oxides account for smaller quantities.


TABLE 8.1.2 EXAMPLE: SETTLING LAGOON FILL TIMECALCULATION

Settling Lagoon Data:Area 5 5 acresDepth 5 5 ftFlow 5 10 million gal/day (mgd)Influent concentration 5 1000 mg/lEffluent concentration 5 50 mg/lSludge density 5 5%

Carbon deposited per day:

(1000 2 50) 3 10 3 8.34 5 80,000 lb/day

Lagoon volume:

V 5 5 acre 3 5 ft 5 25 acre-ft 5 8.3 3 106 gal

Solids capacity of lagoon at 5% sludge density:

5% 5 50,000 mg/l 5 0.42 }glbal}

Capacity 5 0.42 }glbal} 3 8.3 3 106 gal 5 3.5 3 106 lb solids

Time required to fill lagoon with sludge:

T 5 }833

.513

04

1l0b

6

/dlbay

}5 44 days

FIG. 8.1.1 Settling lagoon outfall structure.

Water curtains and scrubbers are used to remove solidsfrom foundry stack gases. Wet scrubbers also removeacidic compounds. Scrubber water is treated to neutralizeacids and to remove solids prior to recirculation. Settledsolids are vacuum filtered prior to disposal. Most foundrieshave a number of scrubbers working on different opera-tions, and all effluents are combined and treated together.In grinding and shakeout areas, the scrubber may be ei-ther cyclonic or water curtain, which tolerates dirty feed-water. However, abrasive materials of 1200 mesh shouldbe removed to avoid abrasion of circulating pumps.

For complete solids removal—down to smoke particlesfrom cupola emission gas—high-energy scrubbers such asVenturis are required, which need clean water. Cupolacooling water should also be clean to prevent heat ex-change surface fouling. If water is used for slag quench-ing, a mass of porous particles up to 1 /4 in is produced.These usually float. Casting washing produces a slurrywith 1150 mesh sand. Most of these materials can be sep-arated on a vibrating screen of approximately 50 mesh.

Depending on the recirculation system, grit separators,settling basins, or clarifiers are used. A hydroseparator re-moves fine sand down to approximately 50 m. Removalof finer solids requires chemical treatment with lime, alum,and possibly a polyelectrolyte to produce clarified effluentcontaining 10–20 mg/l of suspended solids. Disc, drum, orbelt filters are used for dewatering foundry waste solids.Filter rates range from 25–40 lb of dry solids/hr/sq ft.

Some foundries have sand scrubber wastes. This differsfrom dust collection water as it settles more slowly.Overflow rates of no more than 0.3–0.5 gpm/sq ft can beused. Filtration rates for sand scrubber wastes vary from3–10 lb of solids/hr/sq ft.

Laundry WastesTHE PROBLEM OF COMMERCIALWASTE

Commercial coin-operated laundry installations poseproblems when sewers are not available, and septic tankor leach field systems are utilized. Because of the smallamount of land available for liquid waste discharge, ad-ditional treatment is necessary. Treated effluent reuseshould also be considered.

Table 8.1.3 indicates typical waste flow (Flynn andAndres 1963) from laundry installations on Long Island,N.Y. A typical installation of 20 machines produces 4,000gpd. Depending on soil conditions, this volume might re-quire a much larger disposal area than is available. Table8.1.4 describes typical laundry waste properties and com-position as resembling weak sewage with the exception ofhigh alkyl benzyl sulfonate (ABS) and phosphate contents.

Large quantities of water are required for washing,therefore alleviating both water supply and waste disposal

problems via partial or complete recycling of treated waste-water effluents should be considered.

TREATMENT SYSTEMS

Septic Tanks

Septic tanks followed by leach field systems are often in-adequate to process the quantity and quality of water tobe disposed.

Physical Methods

All laundry waste should be strained in a removable bas-ket so that lint does not clog pumps and other equipmentin the treatment system.

Plan settling of laundry waste removes the heavier gritparticles washed out of clothes. Most biological oxygendemand (BOD) is soluble, therefore settling has little ef-fect on the BOD and chemical oxygen demand (COD) ofthe waste.

Several types of filtration units are used to treat laun-dromat wastes. A sand filter efficiently removes particu-late matter. Pressures and filters usually require less spacethan gravity sand filters. The latter is used following othertreatment methods and is little different from filtrationthrough soil. Filtration through diatomaceous earth filtercake is highly recommended, since it removes bacteria andsome viruses, and is particularly effective in separatingchemical sludges. In diatomaceous earth filtration, priorsettling or sand filtration lengthens filter runs but will notresult in a better quality effluent.


TABLE 8.1.3 TYPICAL WASTE FLOW FROM A COIN-OPERATED WASHING MACHINE

Average wastewater flow 89–240 gal/dayMaximum average flow 587 gal/dayMinimum design basis for

treatment based on a 12–hr day 550 gal/machine

TABLE 8.1.4 TYPICAL QUALITY OF LAUNDRYWASTES

Concentration, mg, per liter

Parameter Average Range

pH 7.13 5.0–7.6BOD 120 50–185COD 315 136–455ABS (methylene blue active 33 15–144

substance)Total Dissolved Solids 700 390–1450Phosphate (PO4

32) 146 84–199Acidity as CaCO3 91 73–124Alkalinity as CaCO3 368 340–420

Chemical Methods

Coagulation or precipitation followed by settling and/orfiltration has proven effective in treating laundromatwastes. Alum alone at a pH of 4–5 may result in a 75%reduction in ABS and an 85% reduction in phosphate con-tent of the waste. Iron salts effect a similar reduction,whereas calcium chloride can reduce ABS by 85%, butthis results in only a 50% reduction in phosphate contentat high doses.

In addition, ABS may be completely neutralized, us-ing a cationic detergent. Tests must be performed to pro-vide exact equalization with no excess of either deter-gent. Substances to perform this are commerciallyavailable. Phosphates are effectively removed by precip-itation techniques. Alum, iron salts, and calcium salts athigh pH offer a high degree of phosphate removal. Betterthan 90% phosphate removal can be obtained by cal-cium chloride combined with adjusting the pH to 10, orby lime, both followed by filtration in a diatomaceousearth filter.

Physicochemical Methods

Considered a physicochemical process, ion exchange hasnot been successful in producing high quality water forreuse from laundry waste.

Residual organic matter may be effectively removed bycontact with activated carbon. Granular carbon in an up-flow pressure tank seems to be most efficient, althoughadding powdered activated carbon to other chemicals priorto filtration can also be effective. Activated carbon is alsoeffective in removing anionic detergents. However, highABS concentration exhausts the capacity of activated car-bon to remove other organic matter, therefore prior treat-ment to reduce ABS should be applied.

Biological Methods

When soluble organic material is present, it is difficult toreduce BOD by more than 60% through chemical pre-cipitation and filtration. To achieve high degrees of BODremoval, biological treatment may be required. Althoughthere is an adequate bacteria food supply of carbon andphosphorus in the waste, total nitrogen content may bedeficient for biological treatment.

Solids Disposal

Chemical precipitation solids and diatomaceous earthsolids are amenable to landfill disposal. Biological sludgesare treated similarly to septic tank sludges. The sludgeholding tank should be conveniently located for periodicpumping by a local scavenging firm.

Suggested Treatment System

A schematic flow diagram for a suggested laundromatwaste treatment system is shown in Figure 8.1.2. Afterscreening lint, waste is stored in a holding tank to equal-ize flow and provide sufficient volume for operating thetreatment system during normal daytime hours. A pumpcan deliver waste to the chemical mixing tank where theappropriate chemicals are added. A settling tank removesthe bulk of precipitated solids prior to diatomaceous earthfiltration. A pump is required to provide pressure for fil-tration in the diatomaceous earth filter. Recycling to thechemical mixing tank would be required during the filterprecoat operation.

Following filtration, activated carbon adsorption maybe practiced as needed. A final storage tank is providedfor adding chlorine if needed or for holding effluent forfuture use. Settling tank sludges and diatomaceous earthfilter discharges should be collected in a sludge holding


FIG. 8.1.2 Laundry waste treatment

tank and pumped out periodically by a scavenger system.This system should provide effluent satisfactory for dis-charge or partial reuse.

QUALITY OF EFFLUENT

Chemically precipitated and filtered wastes can be disposedin a subsurface system, provided that there is adequateland to accommodate the hydraulic load. Biological treat-ment may be necessary to improve water quality beforedischarge into a small stream.

Water reuse should be considered because of the largevolume. Since chemical coagulants increase total dissolvedsolids in water, complete reuse and recycle would contin-uously increase total dissolved solids. Thus, chemicalsshould be limited to prevent excess. Because the water isstill warm, heat energy can be saved by recycling treatedeffluent. To control total solids buildup, an ion exchangesystem is theoretically applicable. However, experienceshows that this system is not effective in treating laundrywaste effluents. Other uses for the treated water may befound, depending on the water requirements of nearby in-dustries. Recharging water into the soil uses the soil’s nat-ural treatment ability and maintains a high water level inthe aquifer, providing water for the laundromat.

Mill ScaleThis is a case history of the design, construction, and op-eration of a wastewater treatment system established toremove mill scale from water contaminated by steel millscale removal operation and to provide a closed systemenabling reuse of water for the mill scale removal opera-tion.

The installed cost of the total system was approxi-mately $600,000, including two parallel treatment sys-

tems assuring continuous 24-hr operation via available al-ternate flow patterns for necessary equipment repair ormaintenance.

DESIGN PARAMETERS

To define the problem, existing system elements were re-viewed (Figure 8.1.3). The original design specified a once-through system capable of processing an existing flow of3500 gpm with the capability to handle 7000 gpm in thefuture. Effluent quality was to meet stringent state re-quirements for discharge to the waterway. Applyingknowledge of stream quality to the original design re-quirements raised question about the once-through con-cept. It was noted that if process utilization of this waterdid not require a higher quality supply than the pollutedraw river water presently used, the need for a once-throughsystem was questionable.

A system to treat this wastewater to meet stage dis-charge standards would be very expensive. However, itcost much less to treat this wastewater only to the extentrequired by the process. Historically, this requirement wasmet by the quality of a badly polluted stream. The costdifference between a reuse system and a once-through dis-charge system is substantial. Water quality design stan-dards were key factors in system cost.

Table 8.1.5 lists the design parameters. Provisions werealso made for sludge and recovered oil handling with min-imal expense and minimal personnel time required. Theoriginal process flowsheet is shown in Figure 8.1.4. Aclosed system of this type is susceptible to three primaryproblems: algal accumulation, dissolved solids buildup,and heat buildup.

Solving these problems requires bactericide and/or al-gicide additives, blowdown and addition of makeup wa-ter, and a system cooling tower. The original design in-cluded a cooling tower hookup, if required, together witha chemical feed system. However, makeup water from the


FIG. 8.1.3 Original water supply layout. A. Original plant wa-ter supply line. (Raw river water was used without pretreatmentfor mill scale removal process.)

TABLE 8.1.5 DESIGN PARAMETERS FOR MILLSCALE WATER TREATMENT PLANTa

Wastewater Flow 3500 gpm existing7000 gpm design capability

Primary Pollutants Iron solids (fines)OilHeat

Treated Effluent Quality Continuous 24-hr reuse Required capability

Acceptable Pollutant Content Iron (suspended solids) in Effluent 600 ppm

Oil 150 ppm (plus freefloating oil)

aSystem to be as fully automatic as possible.

river was thought sufficient to compensate for evaporativelosses and to control dissolved solids buildup. Dissolvedsolids presented no serious problem.

OPERATIONAL HISTORY

In operation, the system is entirely satisfactory. The cool-ing tower was not installed originally because heat lossthrough the system—due to the length of the lines and thesurface area of the tanks—was considered sufficient.During most of the operating time, this was true. However,during summer when ambient surface air temperatures oc-casionally reach 110° to 115°F in this region, Joliet, Ill.,heat loss was not enough to maintain comfort for per-sonnel manning the spray nozzles in the plant. During suchperiods, return water temperature rose to 114°F for a fewdays. Therefore, a cooling tower was installed.

The sludge averages 50 to 60% solids, about the min-imum water content for the sludge to slide easily from thedischarge chutes into catch buckets.

Oil-skimming devices are rotary cylinder units mountedat the water surface level in the tanks. These units requireheat protection to prevent freezing in the winter. Thesludge is recovered; since it consists primarily of mill scale,it can be sold as blast furnace charging material.

Strainers are 0.005 in units with 5,000 gpm capacityeach. These are in the system for insurance in the event ofheavy overloading of the settling tanks. This might occurif one of the two parallel systems was shut down for pumpor ejection mechanism repairs when the mill is operatingat peak capacity.

Until now, the system has performed well, except forminor startup and training problems. Mill operating per-sonnel are pleased, because return water quality is far bet-ter than the raw river water they were using.

Mineral TailingsWastewater from mining or ore beneficiation contains sus-pended particles of fine sand, silt, clay, and possible lime-stone. A large percentage of solids may be colloidal dueto their nature or as a result of milling and flotation pro-


FIG. 8.1.4 Reuse system on steel plant water. (P 5 pump; F5 filter)

TABLE 8.1.6 SETTLING VELOCITY OF SILT ANDSAND PARTICLES IN TERMS OFAPPLICABLE OVERFLOW RATES

Particle Comparable OverflowDiameter (mm) Rate cpm/sq ft

1.0 148.00.4 62.00.2 31.00.1 11.80.06 5.60.04 3.10.02 0.910.01 0.2270.004 0.036

FIG. 8.1.5 Thickener for mineral tailings

cessing with reagents added to disperse the solids. Table8.1.6 shows the velocities at which particles of sand andsilt subside in still water (American Water Works Asso-ciation 1969) at 50°F.

Collodial particles cannot be removed by settling with-out chemical treatment. Because of the chemicals addedin milling and during flotation, it is virtually impossibleto economically clarify mineral tailings, and mineral tail-ing overflows from thickener clarifiers are usually re-tained indefinitely. Figure 8.1.5 illustrates thickener de-sign used in alumina, steel, coal, copper, and potashprocessing.

—E.W.J. Diaper, T.F. Brown, Jr.,E.G. Kominek, D.B. Aulenbach,C.A. Caswell

ReferencesAmerican Water Works Association, Inc. 1969. Water treatment plant

design. New York, N.Y.Anon. 1971. Standard Methods for the Examination of Water and

Wastewater. 13th ed.Aulenbach, D.B., P.C. Town, and M. Chilson. 1970. Treatment of laun-

dromat wastes, Part I. Proceedings, 25th Industrial Waste Conference.Purdue University, Lafayette, Ind. (May 5–7).

Aulenbach, D.B., M. Chilson, and P.C. Town. 1971. Treatment ofLaundromat Wastes, Part II. Proceedings, 26th Industrial WasteConference. Purdue University, Lafayette, Inc. (May 4–6).

Burns and Roe, Inc. 1971. Process design manual for suspensed solidsremoval. Environmental Protection Agency Technology Transfer.

Flynn, J.M. and B. Andres. 1963. Launderette waste treatment processes.J.W.P.C.F., 35:783.

Great Lakes–Upper Mississippi River Board of State Sanitary Engineers.1968. Recommended standards for sewage works.


8.2REMOVING ORGANIC CONTAMINANTS

AldehydesAldehydes have several properties important to water pol-lution control. Saturated aldehydes are readily biodegradedand represent a rapid oxygen demand on the ecosystem,whereas unsaturated aldehydes can inhibit biological treat-ment systems at low concentrations. Aldehyde volatilitymakes losses through air stripping an important consider-ation.

BIOLOGICAL OXIDATION

Aldehyde amenability to biodegradation is indicated byhigh biochemical oxygen demand (BOD) levels reportedby several investigators. At a low test concentration,formaldehyde, acetaldehyde, butyraldehyde, crotonalde-hyde, furfural, and benzaldehyde all exhibited substantialbiooxidation (Heukelekian and Rand 1955; Lamb andJenkins 1952). An olefinic linkage in the a,b position usu-ally renders the material inhibitory (Stack 1957). The lev-els inhibitory to unacclimated microorganisms for acrolein,methacrolein and crotonaldehyde were 1.5, 3.5, and 14mg. per liter (mg/l), respectively, whereas levels for ac-etaldehyde, propionaldehyde and butyraldehyde were 500mg/l or above. Formaldehyde was inhibitory at 85 mg/l.

Bacteria can develop adaptive enzymes to allow bio-logical oxidation of many potentially inhibitory aldehydesto proceed at high influent levels. Stabilization by accli-mated organisms of several organic compounds typical ofpetrochemical wastes has been investigated (Hatfield

1957). For organisms acclimated to 500 mg/l formalde-hyde, approximately 3 hr aeration time was required tobring the effluent concentration to zero. However, efflu-ent organic concentration after this interval was still high,indicating oxidation to formic acid or Cannizzaro dismu-tation to methanol and formic acid. Eight to ten hr of aer-ation were required for the effluent BOD to approach zero.Removals of acetaldehyde (measured as BOD) were froman initial concentration of 430 to 35 mg/l after a 5 hr aer-ation time. Propionaldehyde removals were from 410–25mg/l after five hr. The oxidation pattern of paraformalde-hyde, the polymer of formaldehyde, resembled its precur-sor.

Data collected through Warburg respirometer studiesusing seed sludges from three waste treatment plants(Gerhold and Malaney 1966) showed that aldehydes wereoxidized to an extent second only to corresponding pri-mary alcohols. Only formaldehyde exhibited toxicity to allthree sludges. Branching in the carbon chain increased re-sistance to biooxidation.

AIR STRIPPING

Kinetic data for air stripping of propionaldehyde, bu-tyraldehyde, and valeraldehyde have been presented(Gaudy, Engelbrecht and Turner 1961). Removal of pro-pionaldehyde in model units at 25°C followed first-orderreaction kinetics; removals calculated from residual alde-hyde and residual chemical oxygen demand (COD) analy-ses were parallel, indicating that no oxidation of the acid

occurred. However, at 40°C stripping was not describedby first-order kinetics, and propionaldehyde oxidation toless volatile propionic acid was apparent when removalsmeasured as COD were less than those measured as alde-hyde.

Stripping of butyraldehyde and valeraldehyde at 25°Cdid not follow first-order kinetics, indicating oxidation ofaldehyde to acid may also be occurring. Removals afteran 8 hr aeration time at 25°C and an air flow of 900ml/min/l, were 85% for propionaldehyde and butyralde-hyde, and 98% for valeraldehyde. In a biological systemall three removal mechanisms would exist: biological ox-idation and synthesis, air stripping, and air oxidation. Themagnitude of each means would depend primarily on theactivity of the bacterial culture and the degree of gas-liq-uid contact.

CARBON ADSORPTION

Aldehydes, due to their low molecular weight and hy-drophilic nature, are not readily adsorbed onto activatedcarbon. Typical data from Freudlich isotherm tests of ad-sorbability at various carbon dosage levels are presentedin Table 8.2.1. On a relative basis, aldehydes were lessamenable to adsorption than comparable undissociated or-ganic acids but were more amenable than alcohols (Giusti1971). However, none of the low molecular weight, po-lar, highly volatile materials were readily adsorbed.

Cellulose PulpAll pulp mill effluents contain wood extractives, a highlydiverse, ill-defined chemical group that varies widely ac-cording to wood species and origin. Chemical pulpingwastes also contain hydrolyzed hemicelluloses and lignin,solubilized during cooking. Since various pulp processesvary considerably in mill design and operation, effluentsare extremely diverse.

WASTEWATER VOLUME

Problems arise due to the tremendous volumes discharged(Table 8.2.2). Newer installations recycle process waters.Much market pulp is bleached, with bleach plant dis-charges as large as those from pulping. Since mills with500–1000 ton/day capacity are not uncommon, volumesdischarged at a single point may be abnormally high.

EFFLUENT CHARACTERISTICS

Pulp effluents usually have an abnormal pH, a variableloading of suspended fibrous solids, and an appreciableoxygen demand (Table 8.2.2). Older mills may have evenheavier loadings. Kraft pulping produces alkaline wastes,


TABLE 8.2.1 CARBON ADSORPTION OFALDEHYDES

Aldehyde Removalfrom 1000 mg/lSolution at 5 gm/lCarbon Dose

EquilibriumLoading mg/g RemovalCarbon Level, %

Formaldehyde 19 9Acetaldehyde 22 12Propionaldehyde 57 28Butyraldehyde 106 53Acrolein 61 31Crotonaldehyde 92 46Benzaldehyde 188 94Paraldehyde 148 74

TABLE 8.2.2 EFFLUENT CHARACTERISTICS OF CELLULOSE PULPING WASTESa

Water Volume BOD5a Suspended Solids

Unit Process U.S. gal/ton pH lb/ton lb/ton

Hydraulic debarking 500–10,000 4.6–8.0 5–20 30–50Groundwood 6,500–10,000 6.0–6.5 10–40 15–80Neutral sulfite

semichemical pulping(with recovery) 3,000–20,000 6.5–8.5 30–60 ,10

Kraft pulping 6,000–20,000 7.5–10.0 10–50 ,20Sulfite pulping

(no recovery) 20,000–30,000 2.5–3.5 550–750 150–200Sulfite pulping

(with recovery) 20,000–30,000 2.5–4.0 50–100 40–60Bleaching 20,000–40,000 2.0–5.0 10–25 14–25

aOxygen consumed at 20°C during a 5-day incubation with acclimated microorganisms.

whereas sulfite pulping and bleaching plant wastes areacidic. Chemical recovery is essential in keeping oxygen-depleting materials low. Large calcium bisulfite mill efflu-ents may have oxygen demands equivalent to 2,000,000or 3,000,000 people. Effluents display some toxicity toaquatic fauna, albeit of a low order. Neutral and higherpH value effluents are darkly colored, which is aestheti-cally undesirable and inhibits photosynthesis. In smallerstreams, fish downstream from pulp mill outfalls can havetainted flesh. Odor and taste imparted to receiving waterscan also interfere with the subsequent use of the streamfor drinking water. Wind and wave action can create foamon receiving waters, and inorganic salt content may pre-vent use in irrigation.

METHODS OF TREATMENT

No process can alleviate all pulping effluent problems.Abnormal pH is neutralized with slaked lime, calcium car-bonate or sodium hydroxide, since integrated pulping ef-fluents are usually acidic (Laws and Burns 1960; Charlesand Decker 1970). Settling removes suspended solids ex-cept for some mechanically ground “fines.”

All microbiological oxidation systems reduce pulp ef-fluent oxygen demand, but concurrent removal of acutetoxicity is not related to operating parameters for thesesystems. Microbiological treatment may not completelyremove substances responsible for tainting fish flesh orcausing odor, foam, and taste in drinking water.Microbiological treatment does not remove color, how-ever color bodies can be precipitated by massive lime treat-ment (EPA 1970).

RESEARCH PROBLEMS

Originally, pulping waste treatments were the same asthose used in domestic sewage treatment. Problems arisewith pulping effluents because of their variable nature. Inshort-term microbiological oxidation systems, sludge re-cycling difficulties may occur. Biologists emphasize theneed to remove sublethal toxicity, however the responsi-ble chemical entities are largely unknown, and means ofmeasurement are lacking. Massive lime treatment has tech-nical and economic limitations, and specific informationconcerning unresolved problems is lacking. Thus, a con-siderable impetus exists for in-process changes or newprocesses to minimize current wastewater problems.

Food Processing WastesWater is absolutely necessary in food processing. Throughconservation and reuse, liquid waste is reduced, cutting thepollution load. The National Canners Association has setfour conditions governing the use of reclaimed waters incontact with food products:

1. the water must be free of microorganisms of publichealth significance

2. the water must contain no chemicals in concentrationstoxic or otherwise harmful to man

3. the water must be free of any materials or compoundsthat could impart discoloration, off-flavor or odors tothe product or otherwise adversely affect quality

4. the water appearance and content must be aesthicallyacceptable

WATER REUSE

Historically, water reuse was given little consideration.Water is relatively abundant in nature and reuse was con-sidered hazardous due to bacterial contamination.Contamination potential (Figure 8.2.1) shows that, inwashing fruit, unless 40% of the water is exchanged eachhour, the growth rate of bacteriological organisms be-comes extremely high. To overcome this, other means ofcontrol such as chlorination must be used. The importanceof chlorination in maintaining satisfactory sanitary condi-tions is graphically shown in Figure 8.2.2. When chlori-nation was discontinued, the bacterial count more thandoubled. As soon as chlorination resumed, bacterial countswere again brought under control.

Water conservation can be achieved through counter-flow reuse systems. Figure 8.2.3 outlines a counterflow sys-tem for reuse of water in a pea cannery. At the upper right,fresh water is used for the final product wash before thepeas are canned. From this point, the water is reused andcarried back in successive stages for each preceding wash-ing and fluming (the transport of the fruits by flowing wa-ter in an open channel) operation. As the water flows coun-


FIG. 8.2.1 Effect of rate of water replacement on growth ofmesophilic bacteria at 90°F.

tercurrent to the product, the washing and fluming waterbecomes more contaminated; therefore, it is extremely im-portant to add chlorine. At each stage, sufficient chlorineshould be added to satisfy the chlorine demand of the or-ganic matter in the water.

WATER CONSERVATION

Recently, it was determined that adding citric acid to con-trol the pH of fruit fluming waters reduced water use with-out increasing bacteria. A pH of 4 (Figure 8.2.4) will main-tain optimum conditions with cut fruit, such as peaches.The system not only reduces the total water volume andtherefore the amount of wastewater discharged, but alsoincreases product yield due to decreased solids loss fromsugar and acids leaching. Consequently, total organic pol-lutants in the wastewater are reduced. Flavor and color ofthe canned fruit are also improved because of better solu-ble solid retention.

Closed loop systems, such as the hydrostatic cooker-cooler for canned product, are another conservation

method. The water is reused continuously, with freshmakeup water added only to offset minor losses from evap-oration. Closed loop systems not only conserve water butalso reclaim much heat and can result in significant eco-nomic savings.

It is not the intent of this section to describe the enor-mous array of concepts and ramifications used in the foodprocessing industry to reduce water and waste loads whilemaintaining product quality. Many factors determine thefinal effectiveness of proper water use. For example, toma-toes spray-washed on a roller belt where they are turnedare almost twice as clean as the same tomatoes washed ona belt of wire mesh construction. In another example,warm water is approximately 40% more effective in re-moving contaminants than the same volume of cold wa-ter.

There is a delicate balance between water conservationand sanitation, with no straightforward or simple formulafor the least water use. Each process must be evaluatedwith the equipment used to arrive at a satisfactory proce-dure for water use, chlorination, and other factors, suchas detergents.

ELIMINATION OF WATER USE

Eliminating water in certain operations eliminates atten-dant wastewater treatment problems. Wherever possible,food should be handled by either a mechanical belt orpneumatic dry conveying system. If possible, the foodshould be cooled in an air system. Recent studies by theNational Canners Association in comparing hot airblanching of vegetables with conventional hot waterblanching show that both product and environmentalquality were improved by using air. Blanching, used to de-activate enzymes, produces a very strong liquid waste. For


FIG. 8.2.2 Effect of chlorine concentration on bacterial countsin reused water. A. Chlorine concentration; B. Bacterial counts.

FIG. 8.2.3 Four-stage counterflow system in a pea cannery. A.First use of water; B. Second use of water; C. Third use of wa-ter; D. Fourth use of water; E. Concentrated chlorine water.

FIG. 8.2.4 Effect of pH control on bacterial cell growth.

pea processing, this small volume of wastewater is esti-mated to be responsible for 50% of the entire wasteloadBOD; for corn, 60%; and for beets with peelings, 80%.Preliminary results show a reduced pollution load (Table8.2.3), while improving product nutrients, vitamins, andmineral content.

WASTEWATER TREATMENT

Preprocessing

Proper management of food processing wastes requiresconsideration of individual operations from harvestthrough waste disposal as integrated subunits of the totalprocess. Every effort should be made to eliminate wastesand to avoid bringing wastes from the farm into the pro-cessing plant. Where possible, preprocessing should occurin the field, returning the organic materials to the land. Inthe processing plant, wastewater volume and strengthshould be reduced at each step. This principle applies toall food processing wastes, including fruit, vegetables, meatand poultry, and dairy.

Waste segregation within a plant is important in opti-mizing the least-cost approach to treatment. In a typicalbrewery (Figure 8.2.5), where 3% of the flow contains

59% of the BOD, it is less expensive to treat this smallflow separately than to mix it with the entire plant wasteflow. This is effective when a plant treats its own wastesor releases waste to a municipality with surcharges forhigh-strength waste.

Food processing wastes are amenable to biologicaltreatment, and they frequently provide nutrients essentialto efficient biological treatment. Although various wastetreatment methods are available to the food processor(Figure 8.2.6) there is no simple guide for the most prac-tical and economical method.

Lagoons and Land Disposal Systems

Since food wastes contain suspended and soluble organiccontaminants, they are readily treated in lagoons and landdisposal systems. The lagoons may be complete storageponds, frequently used by seasonal processors for wastecontainment. In four to six months, the waste is stabilized,with up to 90% BOD reduction. If large lagoon acreageis available, aerobic conditions are maintained by limitingorganic loadings to less than 100 lb of BOD per acre perday. When extremely strong wastes are encountered, acombination of anaerobic and aerobic lagoons provides anexcellent means of treatment on less land, since the anaer-obic system may reduce BOD from 60% to 90%, reduc-ing the aerobic lagoon acreage required to achieve desiredeffluent quality.

Anaerobic lagoons are odorous and require an artificialor natural cover. In meat products, the high grease con-tent forms a natural cover. Aerobic lagoons can also causeodors if overloaded and lacking sufficient dissolved oxy-gen. Various mechanical aeration methods have reducedrequired lagoon acreage, but these increase power costs.

Land disposal can be achieved by flooding; however,the most efficient means is conventional farm spray irri-gation equipment. Sandy soil with a high infiltration rateoffers no surface runoff, and no discharge to a receivingstream. Recently, an overland flow technique has been de-veloped as an equivalent of tertiary treatment.


TABLE 8.2.3 HOT AIR VS HOT WATER BLANCHING

Blanching Wastewater COD Produced SS ProducedProduct System gal/ton lb/ton lb/ton

Green peas Hot water 1,000.0 32.70 1.42Green peas Hot air 0.018 Not measured Not measuredGreen beans Hot water 1,710.0 4.70 0.11Green beans Hot air 0.25 0.002 0.0002Corn on the cob Hot water 1,223.0 4.70 0.041Corn on the cob Hot air 0.013 5.6 3 1025 1 3 1026

Red beets Hot water 1,333.0 4.11 0.16Red beets Hot air 0.089 0.001 7.4 3 1026

Spinach Hot water 1,430.0 2.6 3 3 1021

Spinach Hot air 3.6 3 1022 3.0 3 1024 3 3 1027

FIG. 8.2.5 Source and relative strength of brewery wastes

Canning Wastes

The canning industry uses an estimated 50 billion gal ofwater per year to process one billion cases of food. Liquidwaste is normally screened as a first step in any treatmentprocess. Solids from these screens can be trucked away asgarbage or collected in a by-products recovery program.

Food product washing is the greatest source of liquidwaste. The water used is normally reclaimed in a coun-terflow system, with a final discharge high in soluble or-ganic matter and containing suspended solids—much of itinorganic—from the soil. Other wastes come from peelingoperations. The amount of suspended matter varies withthe type of peeling. The type of peeler—steam, lye, or abra-sive—has an effect on the nature of the waste generated.

Normal practices utilize large volumes of water to washaway loosened peelings, creating tremendous suspendedand organic loads in the waste stream. Lye peeling alsogenerates wastewater with markedly high caustic alkalineconcentrations. Equipment for dry lye peeling of fruits andvegetables removes the lye peelings in a semidry state sothat solids can be handled separately without liquid con-tamination.

Raw foods are blanched to expel air and gases fromvegetables; to whiten, soften, and precook beans and rice;to inactivate enzymes that cause undesirable flavor andcolor changes; and to prepare products for easy filling intocans. Little fresh water is added during blanching (8-hrshift), therefore the organic material concentration be-


FIG. 8.2.6 Wastewater treatment maze (for organic waste from food processing industries). The diagram illustrates the many op-tions open to solving waste treatment problems. The best route through the maze is suggested by an engineering study and report.Such a report discloses possible treatment methods, anticipated influent properties, effluent requirements and costs. Most important,the report serves as a mutually agreed-upon criterion with regulatory agencies. Designing a waste treatment system should not beconsidered without such a study and report.

comes high due to leaching of sugars, starches, and othersoluble materials. Although low in volume, blanch wateris highly concentrated and frequently represents the largestload of soluble wastes in the entire food processing oper-ation. The amount of dissolved and colloidal organic mat-ter varies, depending on the equipment used.

The last major source of liquid wastes is the washingof equipment, utensils, and cookers, as well as washing offloors and food preparation areas. This wastewater maycontain a large concentration of caustic, increasing the pHabove the level experienced during food processing.

After cooking, the cans are cooled, which requires alarge volume of water. The cooling water is clean andwarm and should be reused for washing.

Meat and Poultry Wastes

Feed lot, stockyard, and poultry receiving area wastes con-sist primarily of manure, unconsumed feed, feathers, andstraw, together with common dirt and drain water.Pollution can be reduced if solid wastes are not diluted bywater.

In killing operations blood must be collected separatelyand prevented from entering sewer or waste treatment sys-tems, since blood has an extremely high waste strength ofabout 100,000 ppm BOD. In poultry plants, variousprocesses must be isolated to avoid cross-contaminationfrom live birds or wastes of previous operations. As thebird goes through the plant on shackles, feathers are re-moved and flumed away. A major incision is made, en-trails and major organs are pulled out, and inedible vis-cera are discarded in a flowaway flume system. The lungsand other material remaining in the carcass are removedby vacuum suction.

Flowaway systems (for feathers, entrails and offal) cre-ate an increased organic load, and it is desirable to use adry conveying system. Most plants use the flowaway sys-tem as a more convenient and nuisance-free operation.After the offal flowaway leaves the area, it must bescreened in order to remove solids. These solids and wastesfrom other operations are then sent to a rendering plantwhere they are utilized in making chicken feed.

Meat packing houses generate a strong waste. Thesewastes are amenable to treatment, as are poultry wastes.Before releasing processing wastewaters into city sewers orprivate waste treatment systems, screening and grease re-moval should be provided to recover solids for by-prod-uct use. Removal of large solids and free floating grease isalso important to avoid clogging sewer lines and foulingbiological treatment systems.

Dairy Wastes

Among waste generating operations in the dairy industryare receiving stations, bottling plants, creameries, ice creamplants, cheese plants, and condensed and dried milk prod-

uct plants. Wastes include separated milk, buttermilk, orwhey, as well as occasional batches of sour milk. Diversemethods are being explored for reclamation and concen-tration of materials, such as reverse osmosis for whey.Unfortunately, there is no simple economical method toreclaim and utilize these materials as byproducts.Indiscriminate dumping of these materials into sewersshould be avoided, and where possible these extremelystrong wastes should be treated separately or eliminatedby hauling.

Milk wastes are normally treated in municipal plants,since most dairies are located in communities. The wastesare amenable to biological treatment, and screening is com-monly provided; grit removal is sometimes necessary, aswell.

Solid Waste Disposal

Most solid wastes from food processing are generated inprocessing raw materials. Some materials, such as pack-aging, faulty or damaged containers, office or warehousepapers, and refuse from laboratories, should be kept sep-arate from the food solids. Solid food waste is producedin growing and harvesting raw crops, in food processing,and by the retailer and consumer.

Many food processing operations are seasonal and gen-erate large quantities of organic solid wastes in a shorttime. The putrescible nature of the wastes requires quickhandling in utilization or disposal. Land disposal opera-tions—by far the most common method of disposal—mustbe rigidly controlled to prevent odor production and flybreeding. It is apparent that the food processing industrymust recycle and recover more of its by-products.

Utilization of food processing waste as animal feed is awidely used method of disposal. In some areas, seafoodcanning waste is pressed into fish meal for animal feed orinto fertilizer material. Tomatoes are pressed and dehy-drated for use as dog food and cattle food. Pea vines, corn-cobs, and corn husks are also used as feed. Citrus peelwaste may be pressed for molasses, which may then beprocessed, dried, and sold as cattle feed. Certain types ofpits and nutshells have been converted to charcoal.

Other possibilities exist, such as producing alcohol fromfruit wastes and composting fruit waste solids, but usuallyit is much cheaper to dump, landfill, spread on the land,or discharge at sea than to attempt reclamation. There doesnot appear to be much chance of a change in this area un-less prevailing economic conditions can be altered throughnew legal restrictions or some form of subsidy program.

HydrocarbonsA bulk oil handling terminal stores and tranships petro-leum products, petrochemicals, animal fats, greases andfood grade vegetable oils. In addition they often acceptand dispose of ballast wastewaters from marine tankers


that deliver to the terminal or pick up cargo for tranship-ment. A biological treatment system is appropriate becauseof the wide range of physical and chemical characteristicsof the various types of oils and petrochemicals; mechani-cal and/or chemical means of separation and neutraliza-tion are too expensive to install and operate.

The equipment used in the system includes (1) a col-lection system for the wastewater flow; (2) an API sepa-rator; (3) a high-rate oxidation pond (or “aerated lagoon”)with a 150,000 gal capacity; (4) a secondary settling or“polishing pond” with a capacity of 450,000 gal; (5) a re-circulation system; and (6) an 800,000 gal storage tankfor ship ballast holding and for surge flow equalization.

DESIGN BASIS

Biological treatment was chosen because some oils float,some sink, some are “soluble,” and some saponifiable.Thus, a broad-spectrum treatment was required. No mu-nicipal sewerage system was available, therefore the efflu-ent had to meet waterway discharge requirements. Thisspecified effluent concentration limits (mg/l): including bi-ological oxygen demand (BOD) of 20 or less; hexane sol-ubles of 15 or less; suspended solids of not over 25; anda pH range of 6 to 10. In addition, effluent had to be sub-stantially color free. Influent characteristics were as fol-lows:

Average daily flow 20 gpmAverage BOD 400 ppmAverage hexane solubles 300 ppmAverage suspended solids 100 ppmAverage pH range 5 to 12

Maximum aeration requirements were calculated to pro-vide (1) sufficient flexibility to vary input air in responseto extreme pollutant load variations; and (2) excess hy-draulic mixing capacity to increase suspended solids oxi-dation and reduce the volume of sludge accumulating inthe system.

The use of 3–5 hp floating aerators provides a totalavailable oxygen transfer rate of 7.5 lb oxygen per lb of

BOD, according to the manufacturer. Under most termi-nal operating conditions, only two aerators were requiredto provide 95% BOD removal. Sludge accumulation wasbelow 350 lb wet sludge (7 lb dry) per day. The systemhas never had an odor problem.

A recirculating system was established for peak wasteloads in oil handling terminal operations (Figure 8.2.7).The 800,000 gal ballast tank gives an additional ten daysof holding time for recirculation when pollutant loadingsfar exceed design capacity.

OPERATIONAL HISTORY

The BOD of the high-rate oxidation pond (“small pond”)at startup was 2420 ppm (mg/l), and the hexane solublecontent was 2040 mg/l. Both ponds were covered withabout 6 in of floating oil and grease (see Figure 8.2.8 forthe rate of stabilization).

The system was set on a recirculation rate of 50 gpm.Three days later, when the pH showed no further erraticswings, dried bacterial cultures (special species of sapro-phytic and facultative bacteria that consume oil) wereadded to create a biomass specifically for oil and greasereduction. The initial dosage was 5 lb, followed by 1 lb/dayaddition for 14 days. After this initiation, the system was


FIG. 8.2.7 Bulk oil-handling terminal waste treatment system.

FIG. 8.2.8 BOD reduction in ponds as a function of time after startup. (BOD is usually 50% of ODI.)

maintained by the addition of As lb of the dried culturethree times a week. Figure 8.2.9 illustrates initial reduc-tion of the hexane soluble content and continuing controlsince the beginning of plant operation.

The effectiveness of a biological treatment to controloily wastewater is also shown in Figure 8.2.10 where the-oretical and actual performances are compared.

PesticidesSince pesticides enter the aquatic environment in runofffrom agricultural areas as well as from point sources, con-trol must be based on a multiphased approach:

1. Controlled application in minimum quantities over ar-eas where specifically needed

2. Degradation in soil and watercourses3. Removal at plants producing potable water4. Treatment of wastes from pesticide handling facilities

and sewered areas

The various mechanisms for removing pesticides enteringthe environment are discussed in this section as outlinedin Table 8.2.4, and the chemical structures of the pesti-cides are shown in Figure 8.2.11.

PESTICIDE REMOVAL IN NATURALAQUATIC SYSTEMS

Pesticide occurrence in surface waters can be traced to sev-eral sources: agricultural runoff, industrial discharge, pur-poseful application, cleaning of contaminated equipment,and accidental spillage. Chlorinated hydrocarbons in aque-ous solutions are readily adsorbed by clay materials. Afteradsorption, small fractions of some pesticides are gradu-ally desorbed into the overlying water where the pesticideconcentration is maintained at a dynamic equilibrium level.Drainage of clay-bearing waters from agricultural areasrepresents a continuous supply of pesticides to the aque-ous solution. Desorption rates are not significantly affectedby pH, temperature, salt and organic levels (Huang 1971).

The introduction of many new pesticides in recent yearshas created the need for reliable evaluation of the effectson the aquatic biota. The model ecosystem for these eval-uations consists of glass aquaria arranged in a sloping soil-air-water interface (Metcalf, Sangha and Kapoor 1971). Afood chain of plant and animal organisms, compatible withthe environmental conditions simulated in the aquarium,is chosen for following radiolabeled DDT (labeled in thearyl rings with C14) and methoxychlor. Average data pre-sented in Table 8.2.5 show a 13,000-fold increase in con-


FIG. 8.2.9 Polishing pond performance from startup. A 5 ini-tial BOD of 2420 ppm (at startup ODI roughly equals BOD;later BOD is stabilized at 50 percent ODI for this waste); B 5

FIG. 8.2.10 Theoretical vs actual performance. A. Rate of pol-lutant addition reducers; B. Standard theoretical curve for rate ofpollutant reduction by biological treatment systems; C. Curve dis-tortion due to exceptional load condition. System gave 97% re-duction in 30 days.

centration of carbon-14 in the fish over the concentrationin water. The DDE metabolite of DDT was largely re-sponsible for the undesirable accumulations in animal tis-sue noted.

In studies with tritium-labeled methoxychlor, accumu-lations of the pure compound and its degradation prod-ucts in fish were of the order of 0.01 those for DDT(Metcalf, Sangha and Kapoor 1971). The presence of sev-eral degradation products and the relatively low accumu-

lations in most organisms revealed the environmentallydegradable nature of methoxychlor.

The organophosphate insecticides were less persistentin the aquatic environment than were the organochloridecompounds (Graetz, et al. 1970). Depending on environ-mental conditions, degradation is by chemical or microbi-ological means, or both. Chemical degradation involveshydrolysis of the ester linkages. Hydrolysis can be eitheracid-catalyzed, e.g., ciodrin, or base-catalyzed, e.g.,malathion. Microbial degradation can be by hydrolysis oroxidation. Partial degradation is often the case, althoughfor diazinon, chemical hydrolysis of the thiophosphatelinkage attached to the heterocyclic ring results in 2-iso-propyl-4-methyl-6-hydroxypyrimidine, which is degradedrapidly by soil microorganisms. Among the orthophos-phates, parathion is one of the most resistant to chemicalhydrolysis, but microbial degradation to aminoparathioncan proceed.


TABLE 8.2.4 PESTICIDE REMOVAL ORIENTATION

Removal Method Applicability

Adsorption onto clay and precipitates Soils and clay-bearing watercoursesWater treatment coagulation processes

Controlled self-destruction Soil and watercoursesDegradation by biological systems Soil at point of pesticide application

Watercourses receiving runoffcontaining pesticides

Waste treatment system at pesticidehandling facility

Chemical oxidation Water and wastewater treatmentsystems

Activated carbon adsorption Water and wastewater treatmentsystems

Membrane separation Water and wastewater treatmentsystems

Incineration Concentrated residue disposal

FIG. 8.2.11 Chemical structures of key pesticides. A.Chlordane; B. 2,4-D; C. DDT; D. Dieldrin; E. DNOCHP: F.DNOSBP; G. Endrin; H. Heptachlor; I. Lindane; J. Parathion;K. Sevin; L. Silvex; M. 2,4,5-T; N. Toxaphene.

TABLE 8.2.5 DISTRIBUTION OF DDT IN MODELECOSYSTEM

Distribution

Water Snail Fish

Total Carbon-14Content, mg. perliter 0.003 20 38

Distribution, %as DDTa 5 31 31as DDEb 7 47 56as DDDc 8 11 12as polar metabolites 74 7 1Unclassified 6 4 0

aDDT, DichlorodiphenyltrichloroethanebDDE, DichlorodiphenyldichloroethylenecDDD, Dichlorodiphenyldichloroethane

Standard biochemical oxygen demand tests involvingglucose incubation with a carbaryl insecticide, Sevin, indi-cate no inhibition of bacterial oxidation of glucose up toa Sevin concentration of 100 mg/l. In fact, Sevin was bioox-idized to a considerable extent at this level; oxidation wasenhanced after a period of acclimatization.

BIODEGRADABLE REPLACEMENT ANDCONTROLLED SELF-DESTRUCTION

Biodegradable substitutes have been developed for somehard pesticides. One approach is to substitute aromaticchlorine atoms in the DDT molecule (Anon., ChemicalWeek 109:36 1971). The new compounds reportedly donot build up in animal tissue and concentrate at higherlevels in the food chain.

A mildly acid reduction by zinc will speed degradationof DDT and other pesticides in natural systems (EPA1970). A copper catalyst speeds up the reduction. Effectivedegradation of DDT to bis(p-chlorophenyl) ethane appearspossible in soil by using micron-sized particles of the re-ductant in close proximity to the DDT. Thin, slowly sol-uble wax or silyl coatings on the reductant can delay thereaction. A second technique for delayed reaction involvescontrolled air oxidation to sulfur to produce the requiredacidity. Effective degradation of DDT in aqueous systemswas also achieved using reduction techniques. The proce-dure was reported effective in substantially degradingdieldrin, endrin, aldrin, chlordane, toxaphene, Kelthane,methoxychlor, Perthane and lindane.

BIOLOGICAL TREATMENT PROCESSES

The waste flow from a parathion production unit under-goes activated sludge treatment (Coley and Stutz 1966)with a residence time of 7–10 days, providing nearly com-plete breakdown of parathion and paranitrophenol as wellas over 95% reduction in organic matter as measured bychemical oxygen demand (COD).

Studies were also conducted in designing a wastewatertreatment facility for production of organic phosphoruspesticides (Lue-Hing and Brady 1968). Although treata-bility studies showed the waste to be biodegradable, shockloads caused stresses at up to 6000 mg/l solids.Consequently, a two-stage activated sludge system waschosen in which the first stage is a dispensable, low-solids,detoxification unit. Removal of dissolved organic mattermeasured as biochemical oxygen demand was 90–98% inthe pilot plant.

The oxidation of Sevin carbaryl insecticide by an acti-vated sludge culture is depicted in Figure 8.2.12. No ad-verse effects on bacteria, protozoa and rotifers were noted.Biological degradation studies (Leigh 1969) of lindane in-dicated no significant removal of this pesticide from mi-crobial activity following 28 days of acclimatization in sta-

tically aerated cultures. Removals in unseeded controls(reference samples) were approximately 46% while bio-logical removals averaged only 41%. The biodegradabil-ity of heptachlor could not be deduced from similar stud-ies because analyses of aqueous solutions of this pesticideindicated partial degradation to 1-hydroxyl chlordene andan undetermined compound. Removals of as high as99.4% were attained within four days for heptachlor, butvolatilization losses were considered significant.

The degradation of chlorinated hydrocarbon pesticideswas studied under anaerobic conditions (Hill and McCarty1966) such as lake and stream bottoms, lagoon treatmentsystems, and digestion systems. Lindane and DDT wererapidly decomposed, the latter to DDE which degradedmore slowly. Heptachlor and endrin also formed inter-mediate degradation products within short periods. Therate of decomposition of aldrin was similar to that forDDD; only slight degradation of heptachlor epoxide oc-curred, and dieldrin remained unchanged. Anaerobic con-ditions were more favorable than aerobic conditions forpesticide degradation. Sorption of chlorinated hydrocar-bon pesticides was found to be greater on algae than onbentonite or fine sand; the process was partially reversibleand the degree of sorption was inversely related to the sol-ubility of the pesticide.

Lindane was degraded anaerobically in pure culture;only 0.5% of the lindane present after 1 hr incubation wasfound in the reaction mixture after 27 hr incubation(MacRae, Raghu and Bautista 1969). The covalentlylinked chlorine of the lindane molecule was released. A de-tected intermediate product reached a maximum level af-ter about 4 hr incubation and diminished to undetectablelevels after 27 hr incubation.


FIG. 8.2.12 Oxidation of Sevin carbaryl insecticide by accli-mated bacteria.

CHEMICAL FLOCCULATION ANDOXIDATION

Since pesticides are used mainly in unsewered agriculturalareas, they reach lakes and streams without passingthrough treatment facilities. Consequently, ease of removalin conventional water supply treatment processes (whenwater is withdrawn for processing to produce potable wa-ter) is important. A study used pilot water supply treat-ment plants to evaluate conventional and auxilliary treat-ment process effectiveness in removing pesticides fromnatural surface water (Robeck, Dostal, Cohen and Kreiss1965). The results showed that each part of the watertreatment plant had some potential for reducing certainpesticides. The effectiveness of the standard process of co-agulation and filtration is shown in Table 8.2.6. Removalsranged from 98% for DDT to less than 10% for lindane.The only pesticide affected significantly by the applicationof chlorine or potassium permanganate (1–5 mg/l) wasparathion, 75% of which was oxidized to paroxon, a moretoxic material. At high dosages, ozone (10–38 mg/l) re-duced chlorinated hydrocarbons; by-products of unknowntoxicity were formed.

In full-scale evaluations (Nicholson, Grzenda andTeasley 1968), the standard processing steps of coagula-tion, settling, rapid sand filtration, and chlorination weresuccessful in reducing DDT and DDE levels but nottoxaphene and lindane levels. Side tests with a 25-m filterremoved DDT and DDE more effectively than toxapheneand lindane, indicating that the latter materials were trans-ported in solution.

Chemical degradability of frequently used chlorinatedhydrocarbon insecticides has also been investigated (Leigh1969). Lindane and endrin were not removed by eitherchlorine or potassium permanganate at oxidant dosagesranging from 48 to 61 mg/l, contact times of 48 hr and awide range of pH values. Heptachlor was removed byKMnO4 to the extent of 88% with only slight variationdue to pH adjustment. Heptachlor and DDT were bothpartially removed by chlorine, and DDT was partially re-moved by KMnO4 with slightly higher removals at lower

pH levels. Maximum removals by potassium persulfate,attained only for lindane and DDT, were 9.4% and18.5%, respectively, at higher pH values.

Several physical and chemical treatments for removingthe herbicide 2,4-D and its ester derivatives from naturalwaters have also been investigated (Aly and Faust 1965).Chemical coagulation of 1 mg/l solutions by 100 mg/l alu-minum sulfate showed no promise with the herbicides andderivatives studied. Activated carbon studies indicated car-bon requirements for reducing 2,4-D concentrations from1 to 0.1 mg/l were 31 mg/l for sodium salt, 14 mg/l forisopropyl ester, 15 mg/l for butyl ester and 16 mg/l forisooctyl ester. Potassium permanganate dosed at 3 mg/ldid not oxidize 1 mg/l of these same compounds. However,0.98 mg/l of 2,4-DCP was completely oxidized by 1.25mg/l KMnO4 in 15 min. Ion exchange studies indicatedthat strongly basic anion-exchange resins more effectivelyremoved the compounds studied than cation exchangeresins.

Strong oxidants to degrade chlorinated hydrocarbonpesticides (Buescher, Dougherty and Skrinde 1964) havealso been studied. Preliminary studies with lindane andaldrin showed negligible removals with hydrogen perox-ide and sodium peroxide at 40 mg/l dosages and four-hrcontact times. Chlorination had negligible effects on lin-dane, but completely oxidized aldrin, while potassium per-manganate (KMnO4) oxidized lindane to approximately12% and aldrin, fully. Further studies of potassium per-manganate added in varying doses from 6 to 40 mg/l tolindane solution indicated that the excessive time and ox-idant dosages required for removals greater than 40%made this treatment unfeasible. Complete removal foraldrin could be attained in 15 min at 1 mg/l dosage ofKMnO4.

Due to the relatively small fraction of ozone in the airstream used for ozonation, pesticide removals from airstripping were measured, as well as removals from oxida-tion. Up to 75% of lindane was removed by ozonation,whereas aeration alone had no measurable effect. Dieldrinand aldrin were completely removed almost at once, butaeration studies also showed fairly rapid removals.

ACTIVATED CARBON ADSORPTION

Considerable data on the adsorption of several pesticidesand related nitrophenols on activated carbon have beenreported (Weber and Gould 1966). Carbon loadings of40–53% indicate economic feasibility for removal of tracequantities of these persistent compounds. Rate andLangmuir equilibrium constants for the pesticides areshown in Table 8.2.7. The quantity of pesticide adsorbedper gm of carbon at complete monolayer coverage of thecarbon surface (Xm values) indicates high ultimate carbonloadings. B21 values, which relate to energies of adsorp-tion, indicate that relatively high residual concentrations


TABLE 8.2.6 REMOVAL OF PESTICIDES IN WATERTREATMENT PLANT OPERATIONS

Removal, percent

Pesticide Coagulation-Carbon Slurry

(10 ppb dosage) Filtration 5 ppm 20 ppm

Lindane ,10 30 80Endrin 35 80 94Dieldrin 55 75 922,4,5-T Ester 65 80 95Parathion 80 .99 .99DDT 98 Not Not

Tested Tested

are required for all but parathion to attain saturation ca-pacity.

Additional studies (Dedrick and Beckman 1967) indi-cate that adsorption of 2,4-dichlorophenoxyacetic acid(2,4-D) can be correlated by both the Freundlich and theLangmuir isotherms; however, two sets of correlating con-stants are required for each of the low and high concen-tration ranges. No significant differences in carbon ca-pacities were noted between granular and powderedcarbon. Carbon loadings of approximately 60% by weightof the herbicide were attained at liquid concentrations95% of saturation, or about 740 mg/l.

Carbon adsorption studies using a slurry approachshowed parathion to be most amenable and lindane leastamenable (Table 8.2.6) to removal by activated carbon.Use of a granular bed at 0.5 gpm/cu ft resulted in almostcomplete removal of all pesticides.

REVERSE OSMOSIS

Specific chemical permeation through a cellulose acetatemembrane has also been reported (Hindin, Bennett andNarayanan 1969). The membranes were immersed in wa-ter at 82°C for 30 min prior to use. At a pressure differ-ential of 100 atm, a temperature of 25°C, flux rates onthe order of 15 gal/sq ft/day, and feed concentrations ofabout 500 mg/l, reduction of lindane was 73% while DDTand TDE (DDD) were rejected above 99%. High reduc-tions were obtained for those chemical species existingprimarily in the colloidal, aggregate, micelle, or macro-molecular form. If the chemical species existed both as anaggregate in dispersion and as a discrete molecule in truesolution where vapor pressure of the discrete molecule intrue solution was appreciably greater than that of water,the range of reduction was 50–80%. Where discrete mol-ecules more volatile than water were tested, range of re-ductions was 14–40%.

INCINERATION

Along with deep-well injection, incineration of concen-trated pesticide waste is an alternative to treatment anddisposal in surface waters. Solid wastes are burned in a ro-tary kiln or other incinerator at 1600°–2200°F (Anon.Chemical Week 108:37 1971). Afterburners can be usedto reach temperatures of 2800°F. A scrubber is used toclean exhaust gases.

RESEARCH TRENDS

Since outlawing DDT and other pesticides that build upin the foodchain seems imminent in many developed ar-eas, replacements must be found, or there will be a re-crudescence of health problems. For example, malaria andVenezuelan equine encephalomyelitis resurge in areaswhere mosquito control is lax or mosquitos become re-sistant to the pesticides used. In the case of mosquito con-trol, malathion and propoxur are recommended as re-placements for DDT as resistance grows (Anon. ChemicalWeek 109:36 1971). Although fenitrothion, iodofenphos,phenothoate and Landrin show promise, all are more ex-pensive and less effective than DDT.

Until suitable replacements are developed, much re-mains to be done in the realm of pesticide removal fromwaters—both prior to discharge of wastewater and intreating water for human use. Although the literature onthe effects and measurement of pesticides is voluminous,articles on removal techniques for pesticides are relativelyfew.

PhenolAlthough phenol (C6H5OH) has been detected in decay-ing organic matter and animal urine, its presence in a sur-face stream is attributed to industrial pollution. Petroleumrefineries, coke plants, and resin plants are major indus-


TABLE 8.2.7 CARBON ADSORPTION CONSTANTS FOR ORGANIC PESTICIDES

Relative RateConstant Limiting Monolayer b21 (relates to(mmoles/g)2 Carbon Loading (Xm), energy of adsorbtion)per hr 3 1024 mg per g mg/l

2,4-D 1.44 387 2.322,4,5-T 1.00 448 1.71Silvex 0.71 464 1.86DNOSBP 1.35 444 1.39DNOCHP 1.12 500 1.81Sevin 1.64 — —Parathion 1.49 530 0.24

Note 1: Experimental Conditions: C0 5 10 mmoles per liter, 0.273 mm. Columbia carbon, 25°C

Note 2: Symbols relate to Langmuir isotherm: x 5 }1X

1mb

bCC

}

(Reprinted with permission, from I.C. MacRae, K. Raghu, and E.M. Bautista, 1969, Nature 221:859.

trial phenolic waste sources. Phenolic compounds and theirderivatives are used in coatings, solvents, plastics, explo-sives, fertilizer, textiles, pharmaceuticals, soap, and dyes.

Treatment methods for phenol removal include bio-logical (activated sludge, trickling filter, oxidation pond,and lagoon); chemical oxidation (air, chlorine, chlorinedioxide, ozone, and hydrogen peroxide); physical (acti-vated carbon adsorption, solvent extraction, and ion ex-change); and physicochemical (incineration and electrolyticoxidation).

SOLVENT EXTRACTION

For wastewaters containing high phenol concentrations,solvent extraction reduces the phenol to acceptable levels.Occasionally, recovered phenol is reused in the manufac-turing process or solid as a by-product. In solvent extrac-tion, two immiscible or partially soluble liquids arebrought into contact for transfer of one or more compo-nents. Using a solvent such as benzene, phenol can be ex-tracted from the wastewater. The extracted phenol is thenwashed out with caustic to form the sodium salt, and thebenzene is reused. In the petroleum industry, light catalyticcracking oils are used as extractors, and in the coking in-dustry, coke oven light oils are used as extractors. Processefficiency depends on solvent choice and system design.

BIOLOGICAL TREATMENT

The microorganisms capable of degrading phenol arehighly specialized and require a controlled, stable envi-ronment. Under ideal conditions several weeks are requiredto develop the proper biological sludge. The efficiency ofan acclimated biological system treating phenolic wastesdepends strongly on temperature, pH, nutrients (nitrogen,phosphorus, minerals), oxygen concentration, phenol con-centration, and other organics concentrations in the waste-water.

To degrade phenol, the microorganism population mustbe stable. Fluctuation in any of the preceding variablesshifts the balance of this population, reducing system effi-ciency and possibly killing the biological organisms.Optimum phenol removal occurs at neutral pH (7.0), 70°Fand constant phenol concentration.

Biological methods of phenol removal include activatedsludge, trickling filters, oxidation ponds, and lagoons.Efficiency ranges from 65–90% removal, depending on theability of the particular wastewater treatment system tocontrol the process variables listed. Activated sludge, trick-ling filters, and oxidation ponds are all capable of highphenol removal if properly designed and operated; how-ever, the trickling filter process is regarded as being morecapable of withstanding slug loads without loss of perfor-mance. Lagoons for treating phenolic wastes are designedto avoid overflow, with evaporation and seepage used to

balance the influent flow. This method is less desirable,due to the possibility of ground water pollution, odor, andoverflows from rainfall.

Frequently, phenolic wastes are diluted with sanitarywastes and treated at the local municipal plant (Mullerand Covertry 1968). Combined municipal-industrial treat-ment buffers the dilution and provides an ample supply ofnutrients and microorganisms should the system be upset.Phenolic wastewaters should be neutralized prior to dis-charge to the municipal sewer system.

CARBON ADSORPTION

Activated carbon in the powdered and granular forms isused to remove phenolic tastes and odors from drinkingwater supplies. In wastewater treatment applications,where phenol content is considerably greater than inpotable water applications and the flow is continuous,granular carbon systems are more economical.

Depending on the concentration of phenol and otherorganic compounds in the wastewater, activated carbonwill adsorb from 10 to 25 lb of phenol per 100 lb of car-bon. This capacity can be determined from isotherm andcolumn test data. In general, phenol adsorption improvesas the pH decreases.

Adsorption at high pH is poor, since phenolate saltforms and is difficult to adsorb. This is an advantage inapplications where phenol recovery is worthwhile. Thephenol is adsorbed at the low pH and reclaimed as sodiumsalt by chemical regeneration, using hot caustic. If the phe-nolate cannot be reused, regenerant disposal is a problem.Also, if quantities of other organic substances are presentin the waste stream, they too will be adsorbed. These or-ganic compounds may not be desorbed during caustic re-generation, which will decrease the phenol capacity of thecarbon upon subsequent regeneration. If chemical regen-eration does not sufficiently recover the phenol capacityof the carbon, thermal reactivation will be required.

Figure 8.2.13 is a flow diagram of a granular carbonsystem for phenol removal employing chemical regenera-tion and phenol recovery. Pretreatment consists of acidifi-cation to pH 4.2 to precipitate the suspended solids andclarify the overflow. The phenol content of the feedstreamranges from 400 to 2500 mg/l, and the effluent objectiveis less than 1 mg/l phenol (Gould and Taylor 1969).

CHEMICAL OXIDATION

Air, chlorine, ozone, and other chemical oxidizing agentsare used to destroy phenol, which is first converted to hy-droquinone and then to quinone. Additional oxidation de-stroys the aromatic ring, forming organic acids and even-tually carbon dioxide and water (Eisenhauer 1968).

Air is an inexpensive oxidizing agent but reactions areslow. Phenol can be completely decomposed by chlorina-


tion at pH 7.7, provided that the stoichiometric amountof chlorine is added. This is accomplished in water treat-ment plants by superchlorination. The major portion ofthe chlorine applied consumes other organic compoundsand destroys ammonia. Approximately 42 parts of chlo-rine per part of phenol are required (Ohio River ValleySanitation Commission 1951).

Ozonation effectively oxidizes phenol. However, theinitial cost of producing ozone is high. Ammonia does notinterfere in ozonation, and approximately 5.8 parts ofozone are required per part of phenol (Ohio River ValleySanitation Commission 1951).

StarchStarch wastes are produced by food processing operations,including starch manufacturing from corn, potatoes, andwheat. The wastes are essentially carbohydrates with ahigh oxygen demand.

BIOLOGICAL TREATMENT

Starch wastes respond to biological treatment using trick-ling filters, aerated lagoons, or activated sludge processes.Waste pH should be adjusted to between 6.0 and 9.0, sus-pended solids should be removed and, if necessary, nutri-ents should be added to maintain a BOD-nitrogen-phos-phorous ratio of 100 to 5 to 1.

Starch is almost completely oxidized biologically, pro-vided that the loading is maintained within the limits ofthe biological activity. If an activated sludge process is used,it is important to maintain an F to M (BOD to mixedliquor suspended solids) ratio of less than 0.3 (per day) tominimize propagation of filamentous organisms that in-terfere with solids separation.

Oxygen Requirements

In activated sludge operations it is necessary to supply oxy-gen to sustain the process and to provide intimate mixingand contact of activated sludge with the organic matterand nutrients. (A low-speed turbine-type surface aerator isshown in Figure 8.2.14.) Oxygen requirements depend onBOD removal and on process loading. The oxygen re-quirement is expressed by equation 8.2(1):

lb of oxygen required per lb BOD removed

5 A 1 1B 3 28.2(1)

In equation 8.2(1), “A” is related to the oxygen require-ment for synthesis of new cells, and “B” is related to theoxygen requirement for respiration. The value of “A”ranges from 0.35 to 0.55, and “B” ranges from 0.05 to

lb mixed liquor volatile suspended solids}}}}}

lb BOD applied per day


FIG. 8.2.13 Granular carbon systems for phenol removal

FIG. 8.2.14 Low-speed surface aerator installation


TABLE 8.2.8 COMPOSITION OF WASTES FROM A SYNTHETIC FIBER FINISH MILL

BODTotal solids BOD avg. % OWFb

pH range range, ppm ppm avga

Rayon processingScour and dye 8.2–9.0 1.012–5.572 2,832 5.7Salt take-off 6.8–6.9 3.388–7.256 58 0.1Waterproof — — 960 1.9

Acetate processingScour and dye 8.3–8.5 1.534–2.022 2,000 5.0Scour and bleach 8.9–9.6 766–946 750 1.8

(Estimated) (Estimated)First rinse 7.0–9.1 108–188 PeroxideSecond rinse 6.8–7.3 80–88 Contained 0.0

peroxide

Nylon processingScour 9.3–12.6 1.492–2.278 1,360 3.4First rinse 8.2–10.7 150–954 90 0.2Second rinse 6.5–8.2 106–932 25 0.1Dye 7.8–9.0 , 318–1,016 368 0.9Last rinse 7.3–7.6 106–134 11 0.0Waterproof — — 450 1.1

Orlon processingFirst scour 9.5–10.0 1.350–2.470 2,190 6.6First rinse 6.4–8.7 102–294 109 0.4First dye 2.2–6.5 , 170–1.950 175 0.5Second rinse 4.1–6.5 116–300 42 0.1Second dye 1.3–1.7 , 130–3.002 995 3.0Second scour 5.9–7.7 , 612–1.824 688 2.0Third rinse 6.3–7.4 82–152 50 0.2Waterproof 3.7–4.3 , 896–2.318 2,110 6.3

Dacron processing(Estimated from OWF concentations as listed)

Scour — — 650Dyes

o-phenylphenol (10% OWF) — — 6,000 18.0benzoic acid (40% OWF) — — 27,000 81.0salicylic acid (40% OWF) — — 24,000 72.0phenylmethylcarbinol (30%

OWF) — — 19,000 57.0monochlorobenzene (40%

OWF) — — 480 1.4

From Masselli, Masselli, and Burford. A simplification of textile waste survey and treatment. New England Interstate Water Pollution Control Commission.a% on weight of fiber, a weight percentage based on dried cloth weight.bOWF, weight percentage based on dried cloth.

0.10. As a general rule, one lb of oxygen is required perlb of BOD removed under conventional activated sludgeoperations with an F to M ratio of 0.3 to 0.5. For aero-bic digestion with an F to M ratio of 0.1, approximately1.5 lb of oxygen are required per pound of BOD removed.

Sludge Production

In the activated sludge process, soluble organic matter isconverted to suspended solids in the form of bacterial cells.

The amount of sludge produced is a function of processloading and of BOD removal. Sludge production can beexpressed within practical limits by equation 8.2(2):

5 A 2 1B 3 28.2(2)



lb of volatile suspended solids produced}}}}}

lb BOD removed

The value of “A” varies from 0.4 to 0.9, and the value of“B” from 0.01 to 0.1, depending on the waste beingtreated. An approximate expression for sludge productionin many treatment applications is given in equation 8.2(3):

5 0.75 2 10.05 3 28.2(3)

Based on conventional activated sludge operations, be-tween 0.5 and 0.6 lb of excess sludge are produced per lbof BOD removed. With aerobic digestion, approximately0.2 lb of excess sludge are produced per lb of BOD re-moved.

Aerobically digested sludge can be dewatered on vac-uum filters with loadings of approximately 1 lb/sq ft/hr.Dewatering excess sludge from conventional activatedsludge operations requires a heat treatment for sludge con-ditioning or a heavy dosage of conditioning chemicals toform a filter cake that will dewater and separate from afilter cloth.

Textile Industry WastesTextile industry wastes are categorized by their source.Man-made fibers constitute approximately 80% of the



lb volatile suspended solids produced}}}}

lb BOD removed

fibers used. Table 8.2.8 lists wastewater compositions fromsynthetic fiber finish mills, and Table 8.2.9 reflects per-formance data of the various treatment methods in re-ducing BOD, SS, color, grease, and alkalinity.

In textile wastes the suspended solids concentration isminute, the BOD range can attain 3000 ppm, and colorcan sometimes reach as high as 3000 APHA color units.Electroflocculation removes most color by electrolyticallyinducing flotation and collection of foam. Thereafter, bi-ological or chemical oxidation can be utilized to polish theeffluent and reduce the BOD to 25—virtually eliminatingcolor. Such textile mill effluent is of sufficient quality tobe recycled and reused.

Viruses and BacteriaBacteria and viruses are removed or killed by disinfectionand sterilization. Disinfection destroys all harmful mi-croorganisms, while sterilization kills all living organisms.Disinfection of drinking water protects public health bypreventing microorganism growth in the pipelines.Disinfection of wastewater treatment effluents protectsmarine life. Sterilization provides water suitable for med-ical and pharmaceutical use. Numerous disinfection andsterilization techniques are available, and Tables 8.2.10and 8.2.11 compare the effectiveness, advantages, and dis-advantages.


TABLE 8.2.9 TREATMENT PROCESS REMOVAL EFFICIENCIES

Normal reduction %

Treatment Suspendedmethod BOD Grease Color Alkalinity Solids

Grease recoveryAcid cracking 20–30 40–50 0 0 0–50Centrifuge 20–30 24–45 0 0 40–50Evaporation 95 95 0 0

Screening 0–10 0 0 0 20Sedimentation 30–50 80–90 10–50 10–20 50–65Flotation 30–50 95–98 10–20 10–20 50–65Chemical coagulation

CaCl2 40–70 — — — 80–95Lime 1 CaCl2 60 97 — — 80–95CO2 1 CaCl2 15–25 — — — 80–95Alum 20–56 — 75Copperas 20H2SO4 1 alum 21–83Urea 1 alum 32–65H2SO4 1 FeCl2 59–84FeSO4 50–80

Activated sludge 85–90 0–15 10–30 10–30 90–95Trickling filtration 80–85 0–10 10–30 10–30 90–95Lagoons 0–85 0–10 10–30 10–20 30–70

Reprinted, from FWPCA. 1967. The cost of clean water, vol. III. Industrial Waste Profile, No. 4. Textile MillProducts. September.

TABLE 8.2.10 DISINFECTION TREATMENT METHODS

Halogens; Metal IonsChlorination (bromine; (silver mer-

Features (using liquid Cl2) Ozonation Ultraviolet Heating iodine) cury, copper)

Required dosagea (ppm) 1–3 (A); 2–5 (B) 1.5–4.0 (A); 2.5–5.0 (B) — — — —

Contact time required (minutes) 10–30 5–10 Minimum 15–20 10–30 120

Bacteria Yes Yes Yes Yes Yes YesEffectiveness against6 Virus Some Yes Some Yes Some No

Spores No Yes No No No No

Advantages Inexpensive and well- Rapid method of removing Fast method Requires no Similar to chlo- Has long-lastingdeveloped technology, color, taste and odor while which requires special equip- rine except less bactericidalwhich provides lasting destroying viruses and no chemicals. ment. irritating to the effect.protective residual. spores; generated on site; eye.

oxidation products are non-toxic.

Disadvantages Not effective against some More expensive and less Leaves no protective Slow and Slower and more Slow and expen-spores and viruses; can, in developed than chlorine residue, expensive, expensive. expensive than sive. Amines andhigh concentrations, produce and it does not leave a not applicable on chlorine. other pollutantsproducts that are toxic to protective residue. large scale and re- interfere with itsmarine life and can cause quires pretreatment effectiveness.undesirable taste and odor. for turbidity re-

moval.

Other Remarks Most frequently utilized Frequently used in Europe; Mostly used on Excellent house- Sometimes used asmethod in the United States. combined with chlorina- special laboratory hold emergency swimming pool —

tion, it can produce high- and small industrial method. disinfectant.quality drinking water. applications

Note: A 5 requirements for drinking water disinfection.B 5 requirements for the disinfection of secondary (activated) wastewaters treatment effluent.

Disinfection should kill or inactivate all disease-pro-ducing (pathogenic) organisms, bacteria, and viruses of in-testinal origin (enteric).

Pathogenic organisms include (1) bacteria of the col-iform group, both fecal and nonfecal, such as Escherichiacoli, Aerobacter aerogenes, and Escherichia freundii; (2)bacteria of the fecal streptococcus group; (3) other mi-croorganisms such as Salmonella, Shigella, and the cystEndamoeba histolytica; and (4) enteric viruses such as theetiologic agents of polio and infectious hepatitis. Test pro-cedures, developed for their identification, are usually in-volved and time consuming. Therefore, the identifications(Metcalf, Wallis and Melmick 1972) of one group of bac-teria (coliform) is usually taken as an indication of waterquality and a measure of effectiveness of bacteria disin-fection. It is assumed that the absence of coliform bacte-ria indicates the absence of all pathogenic bacteria.

Enteric viruses in the drinking water are reported to beresponsible for hepatitis, poliomyelitis, and other epidemicdiseases. Viruses are substantially more resistant to chlo-rine than bacteria, and the absence of coliform bacteriadoes not necessarily indicate the absence of viruses.Virology is not developed to the point that routine iden-tification and assay tests are possible. The development ofa portable virus concentrator, making routine identifica-tion and assay of viruses in water and wastewater morepractical, has been reported. The concentrator first re-moves suspended solids through filtration and absorbsviruses on a cellulose adsorption column. The viruses arethen eluted from the adsorption column and subjected tostandard laboratory assay. (1972).

The probability of disease (D) when a pathogenic or-ganism is brought into contact with a human water con-sumer (host) is proportional to the number of organisms(N) and their virulence (V) and inversely proportional tothe resistance (R) of the host. The purpose of disinfectionis to minimize N and V in equation 8.2(4).

D 5 }NRV} 8.2(4)

Disinfection treatments utilize oxidation, surface activechemicals, acids and bases, metal ions, ultraviolet radia-tion, and physical treatment.

CHLORINATION

Chlorination is by far the most frequently used disinfec-tion method in United States municipal drinking watertreatment plants. The acting disinfectant may be chlorineor a chlorine derivative, such as hypochlorous acid (mostcommonly), chloramines, or chlorine dioxide. Severaltreatment methods have been developed. Simple chlorina-tion involves adding chlorine after filtration or as the onlytreatment. Chlorine-ammonia treatment utilizes the addi-tion of both ammonia and chlorine and the germicidal ac-tion of chloramines. Residual chlorination is applied toprovide residual chlorine in the water. Breakpoint chlori-nation adds sufficient chlorine to react with ammonia andall other chemicals present as well as to assure a free chlo-rine residue.

Liquid chlorine is the least expensive form of chlorine.It was used in most large municipal water works until sev-eral large cities restricted or prohibited transportation andstorage of large volumes of liquid chlorine to prevent ac-cidental release into the atmosphere. Chlorine can be usedand stored more safely in its solid form as Ca(OCl)2.However, the cost is substantially higher.

Bactericidal and Viricidal Action

The bactericidal action of chlorine is the result of its strongoxidizing power. The formation of hypochlorous acid, thestrongest disinfecting agent among the chlorine derivatives,is shown by equation 8.2(5). The bacteria-killing mecha-nism is believed to involve diffusion of hypochlorous acidthrough the cell membrane and oxidation of the cell en-zymes.

Cl2 1 H2O « HOCl 1 HCl 8.2(5)

The viricidal action of hypochlorous acid is substan-


TABLE 8.2.11 STERILIZATION TREATMENT METHODS

OperatingTreatment Conditions Advantages Disadvantages

Heating in 121°C for Reliable Slow heatupautoclaves 15 min

Ozonation 4–5 ppm for Effective against Fast15 min all micro-

organismsSuperchlorination- 5–6 ppm for Effective against Long contact

Dechlorination 2–3 hr most micro- time, need toorganisms; in- dechlorinate afterexpensive treatment

Ultraviolet Fast; no chemical Not effectiveadded against spores

tially slower and less effective than its bactericidal action.The killing mechanism is believed to involve attackingmany protein sites rather than one critical site of the virus.The chlorine treatment, designed to kill bacteria, does notnecessarily kill viruses. Chlorine is not effective in normalconcentrations to kill the cyst Endamoeba histolytica, thecause of amoebic dysentery, a protozoan disease that in-vades the body by a parasitic organism through the in-testinal tract. Fortunately, it is a relatively rare disease.

Chlorine is also ineffective against nematodes, a free-living microorganism present in surface water supplies.Nematodes, although nonpathogenic, are capable of in-gesting and harboring potentially dangerous organisms.

Minimum bactericidal chlorine residual was determinedby the Public Health Service in terms of free available chlo-rine, using a 10-min contact time, and in terms of com-bined available chlorine (free chlorine and chloramines),using a 60-min contact time. The free available chlorinenecessary for disinfection is 0.2 ppm at pH 6–8 and 0.4ppm at pH 8–9. The corresponding concentrations withcombined available chlorine are 1.5 and 1.8 ppm.

OZONATION

Ozone, a triatomic allotrope of oxygen, is produced in-dustrially in an electric discharge field generator from dryair or oxygen at the site of use. The ozone generator pro-duces an ozone-air or ozone-oxygen mixture containing 1and 2% ozone by weight. This gas mixture is introducedinto the water by injection or diffusion into a well-baffledmixing chamber or scrubber, or by spraying the water intoan ozone atmosphere.

Ozone is a powerful oxidizing agent. The mechanismof its bactericidal action is believed to be diffusion throughthe cell membrane followed by the irreversible oxidationof cell enzymes. Disinfection is unusually rapid and re-quires only low ozone concentrations.

The viricidal action of ozone is even faster than its bac-tericidal effect. The mechanism by which the virus is de-stroyed is not yet understood. Ozone is also more effec-tive than chlorine against spores and cysts such asEndamoeba histolytica.

Disinfection, color, taste, and odor control can be ac-complished in a single treatment step by ozonation. Ozonereacts rapidly with all oxidizable organic and inorganicmaterials present in the water.

The ozone dosage necessary for disinfection depends onpollutant concentration in the raw water. An ozone doseof 0.2 to 0.3 ppm is usually sufficient for bactericidal ac-tion only. The ozone dosage necessary for secondary acti-vated wastewater treatment effluent disinfection is 6 ormore ppm. Ozonation leaves no disinfection residue, andtherefore ozonation should be followed by chlorination indrinking water supply treatment applications. To obtainoptimum drinking water, raw water should first beozonated to remove color, odor, and taste and to destroy

bacteria, viruses and other organisms. Then the watershould be chlorinated lightly to prevent recontamination.

Aquarium and Fish Farm WaterDisinfection

Ozonation should be selected as a disinfection treatmentfor marine applications where residual disinfecting agentsor toxic oxidation products (chlorinated amines) cannotbe tolerated. Ozone is unstable in water and decomposesslowly, with a half-life of approximately 30 min at 25°C.The decomposition rate is dependent on water quality. Thehalf-life of the ozone at 25°C is 50 min in distilled waterand 20 min in tap water. Decomposition is substantiallyaccelerated by hydroxyl ions, transition metals and freeradicals. The oxidation products of ozonation are usuallynontoxic and biodegradable. Furthermore, ozonationleaves the water saturated with dissolved oxygen, impor-tant in fish hatcheries or fish farms.

Ozonation disinfects water and saturates it with dis-solved oxygen. Ozonation can reduce organic contami-nants and waste in fish farm water, allowing water recy-cling. Ozone concentrations higher than 0.1 ppm shouldbe avoided because they can harm the fish. Research fromthe National Marine Fisheries Service demonstrates thatozonation destroys undesirable microorganisms with noharmful effects to the fish.

Other Disinfectants. For a discussion of the merits anddrawbacks of ultraviolet irradiation, heating, chemical ox-idants and metal ions, see Table 8.2.10.

—R.A. Conway, C.C. Walden, L.C. Gilde, Jr.,C.A. Caswell, R.H. Zanitsch, E.G.Kominek, J.W. T. Ferretti, L.J. Bollyky

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Lamb, C.R., and G.F. Jenkins. 1952. BOD of Synthetic OrganicChemicals. Proc. 7th Ind. Waste Conference. Purdue University.Lafayette, Ind.

Laws, R.L., and O.B. Burns, Jr. 1960. Recent developments in the ap-plication of the activated sludge process for the treatment of pulp andpaper mill wastes. Pulp and Paper Magazine of Canada61:T507–T513.

Leigh, G.M. 1969. Degradation of selected chlorinated hydrocarbon in-secticides. J. Water Pollution Control Fed. 41(11):R 450.

Lue-Hing, C., and S.D. Brady. 1968. Biological Treatment of OrganicPhosphorus Pesticide Wastewaters. Proc. 23rd Purdue Industrial

Waste Conference, Purdue Univ. Eng. Extension Series, No. 132, 1166.MacRae, I.C., K. Raghu, and E.M. Bautista. 1969. Nature 221:859.Metcalf, R.L., G.K. Sangha, and I.P. Kapoor. 1971. Model ecosystem

for the evaluation of pesticide biodegradability and ecological mag-nification. Environmental Sci. Tech. 5(8):709.

Metcalf, C.J., C. Wallis, and J.L. Melmick. 1972. Concentrations ofviruses from sea water. Proceedings from the 6th InternationalConference on Water Pollution Research. Jerusalem, Israel (June).

Muller, J.M., and F.L. Covertry. 1968. Disposal of coke plant waste inthe sanitary water system. Blast Furnace and Steel Plant. 56(5) (May).p. 400.

Nicholson, H.P., A.R. Grzenda, and J.I. Teasley. J. South-East Sect., Am.Water Works Assoc. 32(1):21.

Ohio River Valley Sanitation Commission. 1951. Phenol wastes—treat-ment by chemical oxidation. (June).

Robeck, G.C., K.A. Dostal, J.M. Cohen, and J.F. Kreissl. 1965.Effectiveness of water treatment processes in pesticide removal. J. Am.Water Works Assoc. 57(2):181.

Stack, V.T., Jr. 1957. Toxicity of alpha, beta-unsaturated carbonyl com-pounds to microorganisms. Industrial and Engineering Chemistry49(5):913.

U.S. Environmental Protection Agency (EPA). 1970. Investigation ofmeans for controlled self-destruction of pesticides. Aerojet-GeneralCorporation Report for the Water Quality Office. 16040 ELO (June).

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8.3REMOVING INORGANIC CONTAMINANTS

AluminumAluminum may be present in acid wastes as the trivalentaluminum ion or in alkaline wastes as an aluminate ion.Aluminum is precipitated as the hydroxide or hydrolysisspecies of polymeric aluminum. The precipitation and con-ditioning of precipitated solids has an important effect onthe separation rate and on the settling and dewatering char-acteristics of the precipitate. Precipitation in the presenceof previously formed solids produces denser and morerapidly settling floc particles. The addition of a polyelec-trolyte also improves settling characteristics. Low velocitygradients, with agitator peripheral speeds as low as 5 ft/sec,are desirable to avoid shearing the floc into small particlesthat settle slowly.

Depending on the method of precipitation and solidsseparation, aluminum hydroxide can be concentrated to1.0 to 2.0% by weight. Adding suitable polyelectrolytes,it can be further dewatered by centrifugation or vacuumfiltration. However, without preconditioning of the alu-minum hydroxide sludge, either precoat vacuum filtrationor filter presses are required for sludge dewatering.

BicarbonateBicarbonate is the principal alkaline form in natural (un-treated) water, although carbonate and hydroxide arefound in lime or lime-soda treated waters, and phosphates

and silicates may also contribute to alkalinity in waste-waters. Adverse effects of high alkalinity in boiler feed-water include corrosion from liberated carbon dioxide andfoaming—with resultant carry-over of contaminants.Scaling can also occur in cooling water systems due to theformation of insoluble calcium carbonate. For drinkingwater, the U.S. Public Health Service Standards limit al-kalinity to 35 ppm over the hardness level.

REMOVING BICARBONATE ALKALINITY

Methods for reducing bicarbonate alkalinity (Table 8.3.1)are divided into chemical addition and ion exchange tech-niques. Chemical methods include cold lime process andacidification. Ion exchange methods involve strong-acidcation exchange, weak-acid cation exchange, or strong-base anion exchange in the chloride cycle.

Cold Lime Process

Bicarbonate is removed by lime addition in accordancewith the following reactions:

Ca(HCO3)2 1 Ca(OH)2 ® 2 CaCO3 1 2 H2O 8.3(1)

Mg(HCO3)2 1 2 Ca(OH)2 ®

Mg(OH)2 1 2 CaCO3 1 2 H2O 8.3(2)

2 NaHCO3 1 Ca(OH)2 ®

CaCO3 1 Na2CO3 1 2 H2O 8.3(3)

As indicated by reactions 8.3(1) and 8.3(2), alkalinity as-sociated with hardness is removed by precipitation.However, sodium bicarbonate is converted to carbonate,which remains soluble and contributes to phenolphthaleinalkalinity, as shown in 8.3(3).

Acidification

By adding sulfuric acid to water, calcium bicarbonate isconverted to calcium sulfate, minimizing scaling by the lesssoluble calcium carbonate. The reaction may be repre-sented as follows:

Ca(HCO3)2 1 H2SO4 ® CaSO4 1 2 CO2 1 2 H2O

8.3(4)

Carbon dioxide formed is removed by aeration. Care mustbe exercised in adding acid to avoid corrosive conditions.With waters already high in sulfate, the solubility productof calcium sulfate may be exceeded, and unwanted pre-cipitation is likely to occur.

Strong Acid Cation Exchange

By passing water through a sulfonic acid cation exchangerin the hydrogen form, the following reaction occurs:

RSO3H 1 NaHCO3 ® RSO3Na 1 CO21 H2O 8.3(5)

(where R represents the resin matrix). Neutral salts areconverted to free mineral acids, and the cation exchangeris regenerated with an excess of acid. The process may be

TABLE 8.3.1 PROCESSES FOR REMOVING BICARBONATE ALKALINITY

Relative RelativeCapital Operating

Process Results, Comments Cost Cost Indicated Application

Cold Lime Process Reduces bicarbonate, calcium and mag- High Low Municipal-large scale, where bicarbon-nesium; increases pH (.10); residual ate hardness exceeds 100–150 ppm,hardness 35–90 ppm; supersaturated 250 gpm and up.CaCO3 may form unless recarbonated.

Acidification and Partial reduction of alkalinity; avoid Low Low Cooling water systems for scale preven-Aeration excess to prevent corrosion; easily tion.

automated; use HCl or HNO3 onhigh-sulfate waters.

Split Stream Process Partial or complete hardness removal; High Moderate Industrial; low-pressure boiler-feed(Strong-Acid Cation alkalinity controlled by proportioning water, especially where ion-exchangeExchange and flow through dealkalizer; excess acid softener already exists; 25–300 gpm.Aeration) disposal; excess salt if softener used.

Weak-Acid Cation Effluent alkalinity from 0–20% of in- Moderate Low Industrial; process and boiler-feed water;Exchange and fluent; “temporary” hardness com- 25–300 gpm.Aeration pletely removed; high acid efficiency,

minimum disposal; may be combinedwith salt-regenerated softener.

Chloride-Cycle Anion Exchange of bicarbonate, sulfate, phos- Low Moderate Small industrial for low-pressure boiler-Exchange phate and nitrate for chloride; no re- feed water; 50 gpm; also municipal—

duction in dissolved solids; regener- where removal of nitrate, phosphateated with NaCl 1 NaOH. and color is required.

© 1999 by CRC Press LLC

used in tandem with a conventional sodium cycle (salt-re-generated) cation exchange softener. Either the raw or soft-ened water is blended with the acidified effluent to give asplit-stream which controls alkalinity.

Weak Acid Cation Exchange

Weak acid resins also remove both alkalinity and hard-ness, as illustrated by the following reaction:

2 RCOOH 1 Ca(HCO3)2 ®

(RCOO)2Ca 1 2 CO21 2 H2O 8.3(6)

In contrast to the strong acid resin, the weak acid ex-changer converts little, if any, of the neutral salts (chlo-rides and sulfates) to free mineral acidity. Neutralizationis not required, although degasification or aeration is prac-ticed. Regeneration of the weak acid resin is more efficientthan with strong acid cation exchangers, minimizing acidwaste disposal. Combining a salt-regenerated softener withan acid-regenerated weak acid resin provides completesoftening and dealkalization with a single column.

Chloride Cycle Anion Exchange

Strong base anion exchangers remove bicarbonate, as il-lustrated by the following reaction:

RCl 1 NaHCO3 « RHCO3 1 NaCl 8.3(7)

The principal regenerant is sodium chloride. Capacities canbe increased by adding a small proportion of caustic sodato the salt. This process eliminates the need for acid-proofequipment and offers convenience, low cost, and a rela-tively small space requirement. In addition to bicarbonateremoval, reductions in sulfate, nitrate, phosphate, anionicsurfactants and color are also achieved. However, the ef-fluent chloride level increases in the exchange.

CadmiumCadmium, a relatively rare element, is extensively used notonly in protecting other metals, but also in manufacturingprimary batteries and standard electrochemical cells; inproducing pigments with outstanding properties; and inproduction of phosphors, semiconductors, electrical con-tactors, and special purpose low-temperature alloys.

SOURCES OF CADMIUM-BEARINGWASTEWATERS

Because the largest consumption of cadmium (60%) is forplating, performed in aqueous baths, there is a drag-outof plating chemicals from the plating bath to the follow-ing rinse bath. The amount of drag-out is a function ofthe size of the article being plated, its intricacy, the pres-ence of blind holes, and the duration of pause to drip overthe plating tank.

Cadmium is a by-product of zinc production and is avaluable source of revenue for the zinc smelter. Duringzinc smelting, evolved cadmium fumes are collected.Consequently, if the gases from electric furnaces, autobodyincineration, and certain domestic products are waterscrubbed, cadmium is found in the scrubbing water.Whenever zinc or brass is electroplated, the drag-out alsocontains cadmium, as these plating tanks serve as cadmiumconcentrators.

In the manufacture, incineration, and careless disposalof primary cells, there is cadmium loss.

The 1962 USPHS Drinking Water Standards set a cad-mium limit of 0.01 mg/l. The toxicity of cadmium and cer-tain disease manifestations necessitate treatment of waste-waters containing cadmium to reduce treated effluentconcentration to the level of 0.01 mg/l.

TREATMENT METHODS

The solubility product of cadmium sulfide is 3.6 3 10229.Its solubility is 8.6 3 10210 mg/l. As cadmium electro-plating is performed in cyanide baths, the drag-out is al-kaline. Therefore, alkaline carbonates and sulfides can re-move cadmium as an insoluble salt. The hydroxide is toosoluble, resulting in cadmium concentrations of 5 to 10mg/l. If carbon dioxide is subsequently absorbed beforeneutralization, additional cadmium will be removed.Cadmium, even when present in trace concentrations, isstrongly coprecipitated with calcium carbonate.

Removal to concentration levels around 0.01 mg/l re-quires the removal of particulate carbonates or sulfides,since residual soluble cadimum can be expected to bewithin limits. The particulates are very small and settlevery slowly, requiring digestion to increase particle size fol-lowed by settling or filtration to remove the fines.

A treatment solution containing NaOH, Na2CO3, Na2Sand CaO will effect satisfactory treatment, but sulfide re-lease may result in disagreeable odors when final effluentpH is reduced to low values, if the sulfide is not destroyed(e.g., by sodium hypochlorite, which is used for cyanidedestruction).

Ion exchange, reverse osmosis, electrodialysis, distilla-tion, and flotation processes can all remove cadmium fromwastewaters.

CalciumCalcium may be present in water solution as bicarbonate,sulfate, chloride, or nitrate. It may also be produced in wa-ter solution when lime is used to neutralize waste acid.

With the exception of calcium carbonate and calciumsulfate, most calcium compounds are very soluble. The sol-ubility of calcium sulfate compounds varies with temper-ature. Gypsum (CaSO4 z 2H2O) has a solublity, in mg/l,of about 1800 at 32°F, 2100 at 100°F, and about 1700at 212°F. The solublity of calcium carbonate in pure wa-

ter is small—about 15 mg/l. However, when precipitated,it produces supersaturated solutions that are relatively sta-ble at water tempertures below 200°F.

Precipitation in the presence of a common ion—eithercalcium or carbonate—reduces solubility. Precipitation inthe presence of about 5% by weight of calcium carbonatevirtually eliminates supersaturation. This same phenome-non is noted when calcium fluoride is precipitated.Precipitated calcium compounds are crystalline and rela-tively easy to dewater by vacuum filtration at rates from20 to 50 lb/sq ft/hr.

ChromiumHexavalent chromium salts occur as pollutants in indus-trial effluents from leather, aluminum anodizing, and metalplating. Chemical plant effluents contain these from ex-tensive use of chromium salts as corrosion inhibitors incooling systems.

In industrial effluents, chromium wastes are treated byreduction and precipitation, removing the pollutant, or byion exchange in which chromate salt is recovered and thedeionized water is reused. The latter treatment recoversthe pollutant chromium for economical reuse.

REDUCTION AND PRECIPITATION

Hexavalent chromium is first reduced to the trivalent stateby adding a reducing agent, with proper adjustment foracidity. This is followed by precipitation of the reducedchromium as the hydroxide, which is then physically re-moved from the system by settling. The reactions are:

ReducingAgent:

Cr61 1 Fe21 1 H1 ® Cr31 1 Fe31 8.3(8)SO2 or SO4

22

Na2S2O5

Cr31 1 OH2 ® Cr(OH)3 8.3(9)

Reaction 8.3(8) proceeds almost instantaneously at a pHof 2.0 or less. Each reducing agent shown in the reactionis effective; Fe21, however, requires an excess of about 21/2times the stoichiometric quantity, resulting in an excess ofFe(OH)3 sludge from neutralization. In small treatmentsystems, sodium metabisulfite (Na2S2O5) is usually the pre-ferred reagent. In water it hydrolyzes to sodium bisulfite,and sulfuric acid must be added to lower the pH for thereducing reaction. Excess reagent must be added if dis-solved oxygen is present in the wastewater. Larger sys-tems, on a batch or continuous basis, use sulfur dioxide,which hydrolyzes to sulfurous acid. Additional acid for pHadjustment is not always required.

ION EXCHANGE

Hexavalent chromium is recovered by ion exchange forreuse as a chromate-rich solution. This solution can be re-

cycled into the cooling tower water treatment system, andthe resulting chromate-free water may be disposed of orfurther demineralized and reused.

A successful process contacts the chromate-ladenwastewater after proper pH adjustment, with a weak-baseanion exchange resin in the sulfate form. The chromate(CrO4

22) ion exchanges with the sulfate (SO422) ion and is

incorporated in the resin. The chromate is recovered as amixture of sodium chromate (Na2CrO4) and sodiumdichromate (Na2Cr2O7) upon regeneration of the resin.The regenerant is a 5% (by weight) solution of causticsoda (NaOH) added in an overall quantity equivalent to10% in excess of the stoichiometric amount.

Sodium hydroxide restores the chromate as sodium saltsand temporarily places the resin in the hydroxyl form. Thesodium hydroxide on the resin is neutralized by adding thestoichiometric quantity of 0.1N sulfuric acid. This neu-tralization step also restores the resin to the sulfate form.The reactions are as follows:

R*H\SO4 1 Na2Cr2O7 WV

R*H\Cr2O71 Na2SO4

R*H“

R*H“

8.3(10)

R*H\Cr2O7 1 NaOH WV (Na2Cr2O4 W V

R*H“

Na2CrO4) 1 2 R°H 2 OH 8.3(11)

2 R*H—OH 1 H2SO4 WV

R*HSO41 2H2O

R*H“

8.3(12)

where:

R° 5 R3N, a weakly basic macroporous resin

A typical flow diagram using the Higgins-type, continu-ous, countercurrent ion exchange system is shown inFigure 8.3.1.

CyanidesThe major portion of cyanide-containing wastewatercomes from metal finishing and metal plating plants.Photo-processing plants also contribute significantly.Cyanides are extremely poisonous, especially at acidic pHlevels, where they are present as hydrocyanic acid, a pow-erful poison. Cyanide-containing wastewater should betreated prior to discharge into sewer lines, streams, orrivers. Treatment processes usually involve either partialoxidation of the cyanide to the substantially less toxiccyanate or complete oxidation to carbon dioxide and ni-trogen. Frequently used oxidizing agents include chlorine,ozone, and electrolytic oxidation. The cyanide concentra-


tion in the effluent should be less than 0.2–1.0 ppm whenthe receiving body is a sewer line, stream, or river.

CHLORINATION

Oxidation of cyanide to cyanate occurs in the pH rangeof 8–9 and requires only minutes (equation 8.3[13]).Further oxidation to carbon dioxide and nitrogen is muchslower, requiring hours (equation 8.3[14]).

NaCN 1 Cl2 1 2NaOH ® NaCNO 1 2NaCl 1 H2O

8.3(13)

2NaCNO 1 3Cl2 1 8NaOH ®

N2 1 2Na2CO3 1 6NaCl 1 4H2O 8.3(14)

Chlorine is added as gaseous chlorine or as a hypochlo-rite solution. Special equipment is required for safe and ef-ficient addition of chlorine gas. For smaller plants,hypochlorite solution is recommended since metering andhandling is simpler and less hazardous. Sludge formationusually accompanies chlorination. The sludge consists ofhydroxides of metal ions, always present in plating solu-tion.

OZONATION

Ozone oxidation of cyanides is best carried out in the pHrange of 9–10, and the oxidation of cyanide to cyanate isextremely rapid (equation 8.3[15]). Further reaction ofcyanates is much slower. The addition of copper (21) saltcatalysts accelerates the reaction. A typical ozone controlsystem is shown in Figure 8.3.2.

CN2 1 O3 ® CNO2 1 O2 8.3(15)

Cyanide oxidation can also be carried out electrolytically.The more toxic sodium and potassium cyanides can also

be converted to substantially less toxic ferrocyanide com-plexes by adding ferrous sulfate. This process is not rec-ommended, however, because ferrocyanide releasescyanide when exposed to sunlight.

Chlorination is the most frequently used and best de-veloped process. The addition of chlorine gas is hazardousand requires storage of large quantities of chlorine. Ozoneis a faster, more powerful oxidizing agent, requiringsmaller holding and reaction tanks. The relative amountsrequired for each process are shown in Table 8.3.2.

FluorideFluoride occurs naturally in some U.S. waters. Dischargesfrom some industrial plants also contain fluoride. The level

FIG. 8.3.1 Chromate recovery by ion exchange.

FIG. 8.3.2 Cyanide waste oxidation control systems utilizingozone as the oxidant. Key: pHRC 5 pH recording controller;ORPR 5 ORP recorder; PC 5 pressure controller; LLC 5 low-level control; AMP 5 amplifier

TABLE 8.3.2 CHEMICAL ADDITIVE REQUIREMENTSOF CYANIDE REMOVAL

Pound ChemicalRequired per lb

Chemical of Cyanide Removed

Chlorine gas 2.7–6.8Sodium hypochlorite 2.9–7.2Calcium hypochlorite 2.8–6.9Ozone 1.8–4.6

of fluoride is primarily of concern in domestic water sup-plies. Data indicate that an average of 1 mg/l of fluorideis beneficial for the prevention of dental caries (the allow-able level of fluoride is determined by the annual averageof the maximum daily air temperature) (U.S. Public HealthService 1962). Higher fluoride levels have been responsi-ble for mottling of teeth. The level of fluoride must alsobe controlled for other uses, such as industrial water sup-ply, irrigation water, stock watering, and aquatic life. Thelimits for these uses in mg/l (McKee and Wolf 1963) areindustrial water (1.0), stock watering (1.0), irrigation (10),and aquatic life (1.5).

Wastewater effluents may contain some fluoride, aslong as adequate dilution is assured in the receiving stream.However, fluoride concentration in effluent is frequentlytoo great to be decreased by diluting waters, requiringtreatment of the waste stream prior to discharge.

Principal flouride removal methods are precipitation bylime, absorption on activated alumina, or removal by anion exchange process. The addition of lime results in theprecipitation of fluoride as calcium fluoride:

2 HF 1 Ca(OH)2 ® CaF2 1 2 H2O 8.3(16)

Precipitated calcium fluoride can be settled out of solutionby thickening and clarification. The settled chemical sludgecan then be treated as other sludges and dewatered utiliz-ing vacuum filtration or centrifugation. The limiting fac-tor for this process is the solubility of calcium fluoride,which is 7.8 mg/l (as F). There are indications that limehigh in magnesium can further reduce the fluoride solu-bility concentration (Rohrer 1971).

Another method of fluoride removal is the use of analuminum compound to bind the aluminum and fluorideas a complex. While filter alum (aluminum sulfate) hasbeen investigated, it has not been effective, as other anionsin the water tend to reduce effectiveness. Activated alu-mina can be used to reduce fluoride concentration to the1–2 mg/l range. The capacity of activated alumina for stor-ing fluoride is about 0.1 lb/cu ft. Flowrates on the orderof 3–5 gpm/sq ft are possible. The activated alumina canbe regenerated with caustic soda, aluminum sulfate, or sul-furic acid with little apparent loss of capacity or activatedalumina volume.

Ion exchange materials have also been investigated fordefluoridation of water. Anion exchange materials regen-erated with a caustic soda solution have been utilized, butthis is an expensive process if fluoride removal is the onlyrequirement.

HardnessHardness is caused by divalent cations (ions with a posi-tive charge of 21). Usually the offending cations are cal-cium (Ca21) and magnesium (Mg21). These and similarcations react with compounds containing monovalent

cations (usually sodium, Na1) to form insoluble products.Along with several lesser problems, these precipitants formencrustations and deposits in hot water pipes, heat ex-changers, and boilers (insolubility and precipitation in-crease with temperature) and also form scum when usingsoap for cleaning.

The effect of soap added to water containing a calciumcompound is most striking. Soap and many calcium com-pounds such as bicarbonate are normally soluble in wa-ter. When the monovalent sodium ion in soap is replacedby calcium, an insoluble end product is formed:

2C17H35COONa 1 Ca (HCO3)2 ®Soap Calcium

bicarbonate

(C17H35COO)2Ca 1 2NaHCO3 8.3(16)Insoluble scum Sodium

bicarbonate

Two types of hardness exist: carbonate hardness and non-carbonate hardness. For the former, the cations are com-bined with either bicarbonate or carbonate. For noncar-bonate hardness, the cations are combined with chlorides,sulfates, and other anions.

ION EXCHANGE

To eliminate hardness, various resins known as zeolites(Ze) are used. These resins usually contain monovalentcations (usually Na1), but since they prefer divalent cationsfor stability, they exchange the Na1 for calcium or mag-nesium.

Ca21 1 Na2Ze ® CaZe 1 2Na1 8.3(17)Sodiumzeolite

The reaction may be reversed by adding a large quantityof monovalent ions (Na1).

LIME AND LIME–SODA ASH SOFTENING

To reduce hardness to 80 or 100 mg/l, the lime or lime–soda ash softening processes may be used. These processesare used when some hardness can be tolerated, as in do-mestic water supplies. The operational cost of theseprocesses is much less than for the ion exchange process.In lime softening, calcium is removed as follows:

Ca (HCO3)2 1 Ca(OH)2 ® 2CaCO3 1 2H2O 8.3(18)Calcium Lime Calcium

bicarbonate carbonate

The calcium carbonate is insoluble and precipitates out. Ifnoncarbonate hardness such as calcium sulfate is also pre-sent, soda ash must be added:

Ca21 1 SO422 1 Na2CO3 ® CaCO3¯ 1 2Na1 1 SO4

22

Sulfate Soda ash8.3(19)

Similar reactions are involved in magnesium precipitation.


Since lime is used in excess, the softened water still con-tains Ca21 and OH2 ions that must be stabilized (Figure8.3.3). This can be done by bubbling carbon dioxidethrough the water (recarbonation). Water softening oper-ations are usually followed by flocculation, settling, andfiltration.

IronIron usually occurs with manganese in groundwater. Thepresence of these metals in excess of 0.1 ppm and 0.05ppm, respectively, is unacceptable for public water sup-plies and for most industrial uses. Above these concentra-

tions, precipitates are formed on contact with air; residuesstain fixtures and interfere with clothes washing and mostmanufacturing processes. The iron may be a water solu-ble ferrous salt or iron bacteria, i.e., hydrated iron oxideenclosed in the cell structure of filamentous microorgan-isms, such as Crenothrix polyspora. Dissolved inorganiciron is usually removed by aeration, chemical precipita-tion, or ion exchange. Iron bacteria removal requires de-struction of cell membranes by strong oxidizing agentssuch as ozone or chlorine (Table 8.3.3).

The oxygen-poor, carbon dioxide–rich lower layers ofwater reservoirs reduce and dissolve iron salts in the soilas ferrous salts. Similarly, the relatively oxygen-free acidic


FIG. 8.3.3 Summary of hardness removal processes.

TABLE 8.3.3 PROCESSES FOR IRON AND MANGANESE REMOVAL

Treatment

Nature of Contaminants Operatingin the Influent Wastewater Processa pH Oxidation Remarks

Iron—no organic matter A-ST-SF 6.5 Yes Easy to operateIron and manganese; little organic matter Acat-ST-SF 6.5 Yes Easy to operate; requires double pumpingIron and manganese bound to organic A-Fcat 6.5 Yes Easy to control; requires double pumping

matter; no excessive organic acids “sniffler” valveIron and manganese bound to organic matter; Fcat 6.5 Yes No aeration but filter reactivated by

no excessive organic acid or carbon dioxide chlorination or by permanganateIron and manganese loosely bound to A-Cl-ST-SF 7.0–8.0 Yes Aeration reduces chlorine requirement

organic matterIron and manganese in combination with A-L-ST-SF 8.5–9.6 Yes pH control required

organic matter and organic acidsIron and manganese in colored turbid A-Co-L-ST-SF 8.5–9.6 Yes Laboratory control required

surface water containing organic matterIron and manganese in oxygen-free well water Cation exchange 6.5 No Periodic regeneration with salt

containing about 1.5 to 2.0 ppm iron and solutionmanganese

Iron in soft well water; iron is present as L-ST-SF 8.0–8.5 No Iron is precipitated as ferrous carbonateferrous bicarbonate in the absence of oxygen

aA 5 aeration; ST 5 settling; SF 5 sand filtration; Acat 5 catalytic aeration; Fcat 5 filtration over oxidation catalysts; Cl 5 chlorination; L 5 lime treatment; Co 5coagulation.

groundwater dissolves iron deposits.

CONTROLLING IRON WITH BACTERIA

Filamentous microorganisms (C. polyspora, Gallinella fer-ruginea and Leptothrix ochracea) thrive in waters con-taining traces of iron and/or manganese. The activelygrowing bacteria precipitate hydrated iron oxide in theircell structures. These bacteria grow in clumps of slime at-tached to pipe walls or other submerged surfaces, causingslow corrosion and dissolution of the iron, thereby plug-ging the pipe. Bacterial growth is controlled by careful re-moval of iron and manganese from the water and by pe-riodic disinfection with chlorine, ozone, or copper sulfate.

REMOVING IRON SALTS

The treatment for removing dissolved iron salts usually in-volves (1) oxidation by air, chlorine, or ozone followed byfiltration; (2) chemical precipitation followed by filtration;or (3) ion exchange. The capacity of the treatment plant,the pH of the water, and the presence of other contami-nants determine which process is the most economical.Iron is usually removed more readily than iron and man-ganese together. The removal of dissolved iron chelated toorganic compounds is usually accomplished by coagula-tion followed by settling and filtration.

Oxidation is accomplished most economically by aera-tion. Aeration also purges carbon dioxide from water,which keeps iron dissolved as ferrous carbonate. Iron ox-ides may be removed by settling, filtration often is neces-sary. If the iron is loosely bound to organic matter, theaeration process is slow and must be accelerated by ironoxide or manganese dioxide catalysts deposited on sand,crushed stone, or coke. Chlorine and ozone effectively ox-idize iron at low pH in the presence of a high organic con-tent.

Chemical precipitation by lime is usually effective if theiron is present as ferric humates. Above a pH of 9.6, mostiron is removed as ferric hydroxide. Treatment is followedby coagulation, settling, and filtration. Ion exchange ef-fectively removes ferrous and manganous salts usingsodium zeolite. Air (oxygen) must be excluded in this op-eration to prevent oxidation to iron and manganese ox-ides, which can form precipitates and plug the ion ex-change column. This process also removes other salts inthe water and decreases hardness.

Aeration is the most economical iron removal methodin large-capacity municipal treatment plants. Chemicalprecipitation is frequently used in beverage and food pro-cessing plants. Ozonation can selectively remove iron andmanganese and preserve the mineral taste of water.

LeadLead is used mainly in various solid forms, as pure metaland in several compounds. Major uses are storage batter-ies, bearings, solder, waste pipelines, radiation shielding,sound and vibration insulation, cable covering, ammuni-tion, printer’s type, surface protection, and weights andballasts.

Wastewaters containing lead originate from only a fewof the processes that produce lead-containing products,such as plating, textile dyeing and printing, photography,and storage battery manufacturing and recycling.

Lead is a toxic, heavy metal limited to 0.05 mg/l byUSPHS Drinking Water Standards, and to 0.10 mg/l byother standards. Discharge standards in sewer use ordi-nances usually limit lead to 0.5 mg/l.

TREATMENT METHODS

Chemical methods of treatment include batch, continuousflow, or integrated with the production process. Table8.3.4 lists several insoluble lead compounds and their cor-responding solubilities at room temperature. The anionsin these compounds and their sources are listed in Table8.3.5.

Aluminum hydroxide from alum use aids in settling thelead sulfate formed. A combination of hydroxide, car-bonate, and sulfide results in a buffered treatment solu-tion, allowing a check on the effectiveness of clarificationdue to the formation of black lead sulfide. Hypochloritecan also be used to prepare the insoluble quadrivalent ox-ide:

Pb21 1 2OH2 ® Pb(OH)2 8.3(20)

Pb(OH)2 1 ClO2 ® PbO2 1 H2O 1 Cl2 8.3(21)

At high pH, lead exists as the plumbate ion, PbO222 which

can also be oxidized by hypochlorite.

PbO222 1 ClO2 1 H2O ® PbO2 1 Cl2 1 2OH2 8.3(22)

In reality, wastewaters contain other substances that also

TABLE 8.3.4 ROOM TEMPERATURE SOLUBILITIESOF LEAD COMPOUNDS

Solubilityin mg.

Compound per liter

Pb Cl2 11.0Pb SO4 4.2 3 1022

Pb (OH)2 1.9 3 1022

Ph CO3 6.0 3 1024

PbCr O4 4.3 3 1025

PbS 4.9 3 10212

Pb SO3 Insoluble2Pb CO3 z Pb(OH)2 Insoluble


require removal. Therefore, a given treatment method mayalso remove other substances, and a treatability study isneeded before selecting a treatment method.

Expected effluent quality in terms of lead concentrationfor batch or continuous flowthrough treatment is reportedto be 0.5 mg/l, whereas that for integrated treatment is0.01 mg/l.

The amphoteric nature of lead compounds requirescareful control of pH for both precipitation and handling(dewatering) of sludges resulting from treatment. Eachstage requires a different operating pH control range. Aswith all precipitation reactions, nucleation and crystalgrowth are important, although the high molecular weightof lead aids in particulate settling.

Physical methods such as electrodialysis, ion exchange,and reverse osmosis can also remove lead from waste-waters. Lead may also be removed by deliberately intro-ducing the wastewater to acclimated biological treatmentplants for complexing with biologically formed organicsubstances. A combination of chemical and biologicalmethods, the in process treatment, can be used, with thelead chemically complexed and removed from the biolog-ical process.

MagnesiumMagnesium is usually present in water or brine as bicar-bonate, sulfate, or chloride. It may also be produced inwater solutions, when dolomitic lime is used to neutralizewaste acid. With the exception of magnesium hydroxide,magnesium compounds are very soluble. The solubility ofmagnesium hydroxide is about 8 mg/l at ambient watertemperatures. However, when precipitated without an ex-cess of hydrogen ion, solubility, including supersaturatedmangesium hydroxide, rises to about 20 mg/l.

If precipitation is carried out in the presence of a highconcentration—up to 5% by weight of previously precip-itated hydroxide—supersaturation is reduced. Magnesiumhydroxide usually precipitates as a flocculant material,which settles slowly and will only concentrate to about1% by weight. However, when precipitated in the pres-

ence of previously precipitated solids, the settling rate anddensity of the settled sludge increase considerably.

Magnesium is not considered a contaminant in waste-water unless it is present in a brine (saltwater). However,concentrations in excess of 125 mg/l can exert a catharticand diuretic effect. In addition, magnesium salts breakdown on heating to form boiler scale.

ManganeseThe limit for manganese in drinking water is 0.05 ppm.Above that concentration it stains fixtures and interfereswith laundering and chemical processing. Manganese usu-ally occurs with iron in ground and surface waters, andmany iron-removing processes (Table 8.3.3) will also re-move manganese. The treatment process usually oxidizesthe water-soluble manganous salts to insoluble manganesedioxide by catalytic air oxidation in the pH range of8.5–10, by chlorination at a pH of 9–10, or by ozonationat neutral pH. Ion exchange is also effective, but it removesother salts that may or may not be desirable.

Manganese, like iron, is extracted from the bottom ofdeep reservoirs or from the ground by carbon dioxide–rich,oxygen-poor water as manganous bicarbonate.

Catalytic air oxidation at an alkaline pH is the mosteconomical method for large treatment plants. Aerationoccurs on contact beds of coke or stone coated with man-ganese dioxide or on beds of pyrolusite. Oxidation is morerapid when the pH is adjusted between 8.5 and 10.0 bylime or caustic (Table 8.3.6). One ppm dissolved oxygenoxidizes approximately 7 ppm manganese. The insolublemanganese dioxide is usually removed by settling and fil-tration.

Oxidation of manganese can also be carried out at aneutral pH using ozone as oxidant, and excessive ozonedosages can oxidize the manganese to pink permanganate.Chlorine oxidation of manganese requires no catalyst.

Ion exchange processes using sodium or hydrogen ex-change resin remove manganous salts effectively, togetherwith other salts. The exchange resin has to be regenerated


TABLE 8.3.5 ANION SOURCES FOR LEADCOMPOUNDS

Anion Source of Anion

Sulfate (SO422) Sulfuric acid or alum

Hydroxide (OH2) Caustic soda or limeCarbonate (CO3

22) Soda ashChromate (CrO4

22) Spent chrome plating bathor chrome plating rinse

Sulfide (S22) Sodium sulfideSulfite (SO3

22) Sulfur dioxide

TABLE 8.3.6 PRECIPITATION OF MANGANESE ANDIRON AS A FUNCTION OF PH

Manganese Iron

Residue Precipitated Residue Precipitated

pH (ppm) (ppm) (%) (ppm) (ppm) (%)

7.0 5.49 0.0 0.0 5.59 0.00 0.08.0 5.49 0.0 0.0 1.53 4.06 72.68.8 5.49 0.0 0.0 0.47 5.12 91.69.0 1.90 3.59 65.49.2 1.00 4.49 81.79.4 0.11 5.38 98.0 0.06 5.53 98.99.6 0.03 5.46 99.4 0.00 5.59 100.0

periodically with sodium chloride or sulfuric acid solution.Aeration at the pH range of 8.5–10 is usually the mosteconomical process for large water treatment plants.Ozonation removes manganese and iron selectively, pre-serving the mineral taste of the water. Ion exchange is thetreatment of choice when both manganese removal andsoftening are necessary.

MercuryMercury exists in water in metallic, mercurous (mercuryI), and mercuric (mercury II) forms, both in solution orsuspension. Removal methods include total and partial re-cycling, impounding, reduction and filtration, sulfide treat-ment, ferrous chloride treatment, and the use of activatedcarbon resins. No treatment to prevent methylation of sed-iments is known. Mercury can be present in solution, sus-pension, and floating, as the metal, soluble, and insolublecompounds, and complexes.

PROPERTIES

Mercury is a silvery liquid; its density is 13.546 at 20°C,its molecular weight is 200.61, its boiling point is 356.9°C,and its freezing point is 238.87°C. It has an appreciablevapor pressure (Othmer and Steinden 1967), and reactswith halogens, sulfur, and oxygen, to give correspondinghalides, sulfides, and oxides. It does not react with water,alkali, or weak acids. It is oxidized by concentrated nitricacid, releasing nitric oxide, and by hot concentrated sul-furic acid, releasing sulfur dioxide. It forms alloys calledamalgams with metals and with ammonium ions. It solu-bility in air-free water is 20–30 ppb at 30°C, but this in-creases in the presence of air, chlorides, and alkali (Linke).

The mercurous ion is the univalent form of mercuryand it is diatomic. It disproportionates in the presence ofsulfide, hydroxy, and cyanide ions (Linke), according toequation 8.3(23).

Hg221 1 S22 5 Hg° 1 HgS 8.3(23)

Mercury(I) salts have low solubility except for the nitrate,chlorate, and perchlorate. They behave as strong elec-trolytes. Mercury(I) is the mercurous or univalent form ofmercury and shows no tendency to form covalent bonds.

Mercury(II), the bivalent form of mercury, forms pre-dominantly covalent bonds and is readily complexed byinorganic and organic ligands such as HgCl4

22, HgS222 and

Hg(CN)422. Mercury(II) forms oxides, sulfides and all the

common salts.Mercury(II) forms organomercury compounds in which

there is at least one C---Hg bond (as distinguished frommercury salts of organic acids and organic complexes). Theorganomercury bond is relatively stable. These compoundsare moderately insoluble in water and may be decomposed

by hydrolysis or oxidation. Metallic mercury and its inor-ganic mercury compounds are converted to organomer-cury compounds as well as the reverse, by geochemicaland biochemical processes.

SOURCES OF CONTAMINATION

The sources of mercury contamination (USGS 1970) aresummarized in Table 8.3.7. Mercury vaporizes from oredeposits and is washed back to earth by rain. The con-centration of mercury in rainwater averages about 0.2 ppb.Mercury is held tightly by the upper 2 in or so of soil.Surface waters generally contain less than 0.1 ppb of mer-cury. Underground waters are higher due to longer con-tact with minerals. Hot springs and exposed ore bodiescontribute higher concentrations in solution and by ero-sion. Man-created sources add to the natural sources.Suspended matter and sediments may contain 5–25 timesas much mercury as the surrounding water bodies.

METHYLATION OF INORGANICMERCURY

Certain microorganisms present in sediments convert in-organic mercury to methyl mercury, which is soluble (5gpl), readily assimilated by aquatic life, and also more toxicthan inorganic forms of mercury (Royal Society of Canada1971). It also becomes more concentrated as it passes upthe food chain.

This biological conversion process was regarded as ananaerobic process but was later found to occur even moreefficiently under aerobic conditions. The process was firstidentified with microorganisms, but higher organisms suchas chickens also seem to have this conversion capacity.


TABLE 8.3.7 SOURCES OF MERCURYCONTAMINATION

NaturalMercury ores, volatilization, solution, and erosion; rocks, vol-

canos, hot springs; rainfall; sediments and methylation of in-organic mercury

Man-CreatedMining and metallurgy; recovery and purification of mercury;

burning fossil fuels; sewage and garbage

IndustrialElectrical apparatus (batteries, lamps, and power tubes); elec-

trolytic chloralkali; paint (mildew proofing and antifouling);instruments (switches, relays, gauges, meters, barometers,thermometers, manometers, and pump seals); catalysts; agri-culture (seed dressing, fungicides); dental preparations; generallaboratory use; pharmaceuticals (diuretics, antiseptics, preser-vatives, and skin preparations); pulp and paper-slimicide; andmiscellaneous

METHODS OF REMOVAL FROM WATER

In nature, soil particles such as clays, oxides, peat moss,and humus adsorb mercury from rainfall and remove itfrom the cycle. The tendency of mercury to sink rapidlyand combine with sulfide in anaerobic bottom sedimentsto form cinnabar (HgS) appears to be a major scavengingmechanism. Reaction of mercury with organic matter isanother such mechanism.

For industrial and metallurgical effluents, the removaltreatment must fit the concentration of mercury and thequantity and composition of effluent. The following stepsand principles apply for preliminary handling:

1. Isolate the mercury-bearing water from mercury-free ef-fluent.

2. Minimize the use and quantity of water.3. Recycle contaminated water. In order to accomplish

this some simple treatment such as cooling, settling, orfiltering may be necessary.

4. Use V-shaped rather than rectangular trenches or floordrains.

5. Use a series of traps to collect solids.6. Impound wastewater in a tank or leak-proof pond for

temporary storage and to even out the variations inflowrates and composition.

7. Observe that pumping underground may be possible insome cases.

8. Settle, filter, or both.

Total Recycle

Recycling mercury-contaminated water is recommended;however, accumulated sludges and/or concentrated so-lutions may be more difficult to treat for mercury re-

moval than the original wastewater. Evaporation wastested and abandoned as impractical because of thevolatility of mercury and the buildup of contaminatedbittern or brines.

Reduction

Reduction of mercury to the metallic state followed by fil-tration may be suitable for small volumes of concentratedeffluent, and can be accomplished by electrolysis, reduc-tion with a less noble metal, or a reducing agent. Mercurycan be recovered in a relatively pure state at the cathodeof a special electrolytic cell. When metallic reducing agentssuch as copper, iron, zinc (New Jersey Zinc Co. 1971),aluminum, and sodium amalgam are used, the mercury isrecovered as an amalgam or as droplets coalescing on thesurface of the metal. Mercury is recovered in the pure stateby distillation. Mercury ions are replaced by the reducingmetal ions according to equations 8.3(24) and 8.3(25).

Zn 1 Hg221 ®2Hg 1 Zn21 8.3(24)

2NaHgx 1 Hg221 ® (2x 1 2)Hg 1 2Na1 8.3(25)

In the latter case, mercury is recovered in the amalgamform.

Reducing agents such as hydrazine, hydroxylamine, hy-pophosphorous, formaldehyde, and sodium borohydride(Anon. Chem Eng. News 48 1970) are suggested. The mer-cury is recovered by coalescence and/or filtration. Mercuryconcentrations remaining in the filter effluent (filtrate) arereported in the 100 ppb range after treatment through a5-micron filter.

FIG. 8.3.4 Sulfide treatment for mercury removal—a continuous process.


Sulfide Treatment

Waste treatment with sodium hydrosulfide (NaHS) orsodium sulfide (Na2S) and a flocculant has been suggestedand widely used as a primary treatment in the chlor-alkaliindustry (Bouveng and Ullman 1969) in the United States(Figure 8.3.4).

Wastewater containing mercury reacts with sulfide ac-cording to equation 8.3(26).

Hg 1 Hg221 1 Hg21 1 2S22 ®2Hg 1 2HgS 8.3(26)

Mercury and mercuric sulfide can be recovered by settlingand/or filtration. However white mercuric sulfide is veryinsoluble and readily forms a soluble complex with excesssulfide.

HgS 1 S22 ® HgS222 8.3(27)

This effect is most severe at high pH—adjustment to apH of 7 or 7.5 is required. Buffering with sodium car-bonate (2 gpl) helps to correct flowrate variations.Automatic control of pH and sulfide concentration is pos-sible using a glass pH electrode and a silver sulfide spe-cific ion electrode. Sulfide is readily oxidized particularlyby chlorine.

Efficient operation gives 50–60 ppb mercury concen-tration in the effluent but under usually occurring upsetconditions, mercury concentration in the effluent increasesto 200–500 ppb.

Ferrous Chloride Treatment

Ferrous chloride reduces mercury salts in wastewaters toinsoluble compounds as shown in equation 8.3(28).

2 Fe21 1 2 Hg21 1 8OH2 ® 2 Fe(OH)3 1 Hg2O 1 H2O

8.3(28)

Wastewater is adjusted to a pH of 9–9.5 and a 50 ppmexcess (over stoichiometric) and FeCl2 is added. After set-tling in a pond for several days, the effluent contains 5–6ppb of mercury.

Adsorption on Activated Carbon

Activated carbon is used as filter-aid and also in granularbeds. Removal performances from 100–200 ppb (influent)to 10–20 ppb (effluent) are reported. For the filtration of50% caustic soda from mercury cells, a precoat type fil-ter with activated carbon precoat, such as Nuchar 722, isused. At the start of a cycle, mercury concentration is 10ppb in the filter effluent, but increases to the Codex(National Academy of Sciences 1971) limit of 500 ppbwhen the filter is backwashed. Mercury is recovered fromthe activated carbon by distillation.

Ion Exchange and Chelating Resins

Anion exchange and chelating resins are commercially usedin chlor-alkali plants in Japan and Sweden (Gardiner 1971;Aktiebolaget Billingsfors-Långed; Ajinomoto Co., Inc.;Terraneers, Ltd.). Untreated influent is first adjusted forpH and oxidation reduction potential and is filtered. Anionresins remove the mercury down to an effluent concen-tration of 100–200 ppb. Chelating resins further reducemercury concentration to 2 ppb. Anion resins and somechelating resins can be regenerated with sodium sulfite,sodium hydrosulfite, or hydrochloric acid. Mercury can berecovered from backwash stripping solutions by electrol-ysis or with sodium amalgam, or the mercury-rich solu-

FIG. 8.3.5 Treatment for mercury removal with ion exchange resin beds followed by a chelating resin bed. Mercury is strippedfrom the ion exchange resin by stripping liquid and then is reduced by sodium amalgam and recovered. A chelating resin is a hete-rocyclic compound that will combine with a metallic ion to form a chelate, i.e., a compound with the metallic ion attached by co-valent bonds to two or more nonmetallic atoms in the same compound.

tion can be used as is. Mercury is recovered from somechelating resins and filter solids by distillation.

The untreated influent wastewater is adjusted to a pHbetween 6 and 8, and chemicals are added to destroy ox-idizing substances (Figure 8.3.5). Then the effluent is fil-tered. The use of an activated carbon filter removes somemercury and destroys oxidants that might exist because offaulty pretreatment. Precipitates may accumulate in theresin beds; consequently, occasional backwashing with wa-ter and air is required.

Counteracting Methylation of Mercury

The discovery that microorganisms in sediments convertinorganic mercury to the soluble and more poisonousmethyl mercury makes it important to inactivate the mer-cury already present in waterways. Suggestions includedredging; covering with fine material (which is effectiveif not disturbed); changing environmental pH and salin-ity; finding a catalyst to change methyl mercury chlorideto dimethyl mercury, which is volatile and would escapefrom the water; and inactivating the sediment with mer-captans.

NickelNickel may be present in plating wastes or in the wastefrom nonferrous metallurgical plants.

Nickel plating is carried out at a pH range of 1.5 to6.0, with the majority of solutions operative between 2.0and 4.5. The concentration of nickel in rinse waters fol-lowing nickel plating varies widely, depending on themethod to minimize drag-out and whether flowthrough orcountercurrent rinsing is used. As a general rule, a three-stage counterflow rinsing operation reduces rinse waterconsumption by 90–95%, making it more economical torecover nickel for reuse.

Nickel may be reclaimed from the rinse tank by evap-oration, and the concentrated solution returned to plating.The condensate is recovered and reused as makeup to therinse system. Ion exchangers are also used for the recov-ery of nickel and water. The rinse water is passed throughcation and anion exchangers in series, with deionized wa-ter recycled into the rinse tanks. The cation exchanger,which removes the nickel ions, is periodically regeneratedwith acid. The regenerating solution containing the con-centrated nickel salts can then be treated and reused inplating operations.

If it is uneconomical to segregate the nickel rinse waterfor recovery, or if waste contains other metals that wouldinterfere with recovery operations, nickel can be com-pletely removed by precipitation with lime at a pH of 8.0or higher. Settling characteristics depend on the techniqueused for precipitation, flocculation, and settling. Designs

should achieve a separation rate of less than 0.5 gpm/sqft, with the clarifier effluent filtered to remove nickel hy-droxide present as suspended solids in the clarifier over-flow.

Precipitated hydroxide can only be concentrated to1.0–2.0% by plain gravity settling. Sludge dewatering re-quires precoat filtration or plate and frame filtration.

SilicaINSOLUBLE SILICA

Wastes with fine sand, silica gels, activated silica sols, andcolloidal silica, as well as silica-containing substances suchas silt, clay, fly ash, diatomaceous earth, diatoms, and min-erals can be removed by clarification or by inline pressurecoagulation-filtration. Without chemical flocculation, fil-tration can only remove 50–80% of silica, or perhaps noneif particle size is sub-m. Macroreticular colloidal removalion exchange resin has been effective in removing virtually100% of colloidal silica (Kunin and Hetherington 1969).

SOLUBLE SILICA

Soluble silica is not an environmental contaminant. It orig-inates from well or surface water supplies and has con-centrations of 1–120 mg/l as SiO2. Above 120 mg/l, col-loidal or crystalline flakes of insoluble silica may be visiblein neutral water (pH , 8.0). Higher concentrations of sil-ica may be solubilized by having a carbonate or hydrox-ide alkalinity of at least 1.5 times the SiO2 concentration.Soluble silica in neutral water (pH 5–8) is present as a mix-ture of bisilicate, HSiO2

3; as weak silicic acid, H2SiO3; andperhaps as SiO2—compounds analogous to HCO2

3,H2CO3, and CO2 in water (Camp 1963).

Removal of soluble silica from wastewater may be re-quired for a few processes (State Water Pollution ControlBoard 1952) or in case of high pressure boiler feed, whena turbine is present (Crits 1968). Soluble silica can be re-moved

1. By warm or hot lime precipitation, with the additionof MgO (or naturally occurring magnesium, if enoughis present) to reduce silica concentration by 80–95%—down to 1.0 mg/l (Applebaum 1968) in the effluent

2. From soft water using a desilicizer, a strongly basic an-ion resin in the hydroxide form (Applebaum 1968;Zunino 1962). Silica concentration in the effluent maybe as low as 0.2 mg/l.

3. By strongly basic anion resins used in demineralization.Silica concentration in the effluent can be reduced to0.002 mg/l if required.

StrontiumStrontium is an alkaline-earth element with a chemistrysimilar to that of calcium and other divalent cations in


Group IIa of the periodic table of the elements. The metalis used in some alloys of tin and lead; various strontiumsalts are used in pyrotechnics, refining beet sugar, glass,paints, ceramics, and some medicines; and strontium ra-dioisotopes are generated from fission reactions at nuclearinstallations (McKee and Wolf 1963). The major liquidindustrial wastes containing strontium are nuclear wastesincluding Sr89 and Sr90.

Strontium removals can be achieved through lime-sodaash softening. When stoichiometric chemical dosages areused, a 65–75% removal efficiency of dissolved strontiumcan be obtained. Increased removals result from chemicaldosages greater than stoichiometric amounts.

Phosphate coagulation can also be utilized since stron-tium cations form relatively insoluble phosphate com-pounds at high pH. At a pH of 11.3 or greater, with aphosphate to calcium ratio greater than 2.2 to 1, morethan 97% of strontium can be removed (Landerdale 1951).Other methods of chemical precipitation include copre-cipitation with aluminum and cesium from acid aluminumwastes, and scavenging by tannic acid or calcium oxalate(Straub 1964).

Inorganic strontium can also be removed from waterthrough cationic ion exchange materials. Processes such aselectrodialysis and reverse osmosis also show promise forstrontium removal application.

SulfateSulfate removal may be required to meet recommendedlimits for drinking water, which suggests a maximum con-centration of 250 mg/l for sulfates and 500 mg/l for totalsolids (U.S. Public Health Service 1963). Sulfate may beremoved at the source for reuse or to prevent downstreambiological reduction that can produce odors.

The choice of a sulfate removal system depends on theinitial sulfate ion content and the final quality of water de-sired. Alternative means of removal are ion exchange,evaporation and crystallization, reverse osmosis and bac-terial reduction.

ION EXCHANGE

Anion exchange resins can be used for removal of sulfateions. For example, a new deionization method (Kunin andDowning 1971) has shown over 9% efficiency in remov-ing waste ions from acid mine waters. Sulfate ions are re-moved by an anion exchange resin functioning in the bi-carbonate cycle according to the following reaction:

2 (R—NH)HCO3 1 FeSO4 ® (R—NH)2SO4 1 Fe(HCO3)2

8.3(29)

Resin regeneration is accomplished with dilute ammoniumhydroxide followed by carbonation with carbon dioxide.

EVAPORATION AND CRYSTALLIZATION

When the sulfate ion content is sufficiently high to war-rant recovery for reuse or sale, a crystallization approachshould be evaluated. Water can be removed by evapora-tion and the sodium sulfate crystallized out (Cosgrove) atabout 5°C as Na2SO4 z 10 H2O (Glauber’s salt). The crys-tals can then be removed by filtration. Metal impuritiescan be removed by neutralizing with sodium hydroxidesolution and refiltering, evaporating, crystallizing, and dry-ing. Depending on the relative concentrations of metal ionsand sulfates, an alternative approach first removes heavymetals by ion exchange.

REVERSE OSMOSIS

Rejection of sulfate ions across a modified, semipermeablecellulose acetate membrane is reported at above 90%(Hindin and Bennett 1969). This process is justified by theneed for water reuse in water-deprived areas and by theavailable means of disposal or recovery of the high sul-fate-containing concentrate.

BIOLOGICAL REDUCTION

Sulfate-reducing bacteria form hydrogen sulfide under re-ducing conditions, i.e., in the absence of molecular oxy-gen and other proton acceptors like nitrate ions. Organicmatter is oxidized to acetic acid by the most common sul-fate-reducing bacteria; however, complete oxidation tocarbon dioxide and water also can occur. If sulfate levelsare high, the objectionable odors resulting from the releaseof hydrogen sulfide gas can be minimized by (1) precipi-tation of metallic sulfide salts; (2) oxidation of the sulfidesto sulfates either by anaerobic photosynthetic bacteria orby microaerophilic sulfur-oxidizing bacteria; and (3) main-taining an alkaline pH level.

TABLE 8.3.8 OXIDIZING WASTEWATERCONTAINING 5 PPM OF H2S

Dosage

TotalWeightof

Dosea OxidantRequired Required

Oxidant (ppm) (lbs/MG)

Ozone 5 41.850% Hydrogen Peroxide 10 83.6Chlorine 40 334.4

aThe dosage requirements differ because ozone is introduced as 1% O3 inair; the oxygen present in air helps to keep the system aerobic, preventing H2Sformation; and the dosage indicated for chlorine is sufficient to prevent furtherH2S formation, while the ozone dosage is not.


SulfideWater containing sulfides in excess of a 0.5 ppm concen-tration has an offensive (rotten eggs) odor and is also verycorrosive. The sulfide is present at acidic pH levels as hy-drogen sulfide gas and at alkaline pH values as sulfide salt.Occasionally, when the water also contains iron, the sul-fide may be present as a finely divided black (FeS) precip-itate. Hydrogen sulfide is usually removed by aeration atan acidic pH, followed by a final oxidation treatment withchlorine, ozone, or hydrogen peroxide.

Aeration at a low pH removes hydrogen sulfide by purg-ing rather than by oxidation. The aerators are usually con-structed of wood rather than metal to avoid corrosion.Bacterial growth aids in sulfide removal. Chemical oxida-tion of 1–2 ppm of hydrogen sulfide in water can be car-ried out most economically with ozone. Hydrogen perox-ide and chlorine are also effective. The oxidant dosagesnecessary to remove 1 ppm hydrogen sulfide are ozone 1ppm, hydrogen peroxide (50%) 2 ppm and chlorine 8ppm. Table 8.3.8 lists treatment for a 1 MGD wastewaterflow containing 5 ppm of hydrogen sulfide.

ZincSpectrographic analysis of 969 river samples by the FW-PCA from 1963 to 1965 showed zinc in 80% of all thesamples. Concentrations (Pickering 1968) varied from0.003 to 1.080 mg/l, with 0.136 mg/l reported as the meanconcentration in drinking water from 37 U.S. locations(Kehoe, Cholak and Largent 1944). In greater than trace

concentrations, zinc is harmful to aquatic organisms. Thetoxic concentration varies with pH and hardness. The 96-hr median tolerance limit (TLm) for fathead minnows is4.7 mg/l zinc concentration with 8 pH and 50 mg/l hard-ness, and it is 35.5 mg/l with 6 pH and 200 mg/l hardness(Mount 1966). Long-term tests on minnows in water with200 mg/l hardness show egg production reduced 50% withthe zinc concentration at 0.009 times the 96-hr TLm value(Brungs 1969), an amount lower than that found in somenatural streams. Soluble zinc concentrations can be re-duced by ion exchange and precipitation processes.

ION EXCHANGE

Using a recycling regeneration process, it is possible to ob-tain an average recovery of 92% (Aston 1968). Hydrogenand sodium ions effect loss of exchange capacity (Blakeand Randle 1967). The loss amounts to 50% when theseions total 15 g/l. On cooling tower water with 6 mg/l zincconcentration, a residual of below 1 mg/l is reported inthe treated effluent. If zinc is being removed for recovery,the ability of ion exchangers to adsorb impurities may ne-cessitate subsequent purification.

PRECIPITATION

Precipitation of the hydroxide neutralizes acid zinc solu-tions and reduces zinc in the same process. If only lime isadded in a single step after extended settling, there is stillless than 1% hydroxide in the sludge. Often such sludgesmust be stored indefinitely in lagoons because they dry


FIG. 8.3.6 Zinc recovery flowsheet (Reprinted from the U.S. Environmental Protection Agency (EPA). 1971. Zinc precipitation andrecovery from viscose rayon wastewater. [American Enka Co. Project No. 12090 ESG. January.])

very slowly. A novel system (Figure 8.3.6) precipitates zincby repeated adsorption on hydroxide particles (Rock1971). These become dense spheroids concentratable to5–10% hydroxide. Residual zinc in the effluent is less than1 mg/l, independent of the zinc concentration in the feed(Chamberlain and Anderson 1971). The zinc is reused ina rayon plant. Other processes obtain improved precipi-tates of zinc carbonate (Courtaulds Ltd.) and sometimesuse inert nuclei. Others reduce the zinc by adsorption andprecipitation in activated sludge systems (Offhaus 1968)if the waste sludge is removed without additional diges-tion.

For the process in Figure 8.3.6 the operating and main-tenance costs for recovery of the soluble waste zinc dependon the sulfuric acid-zinc sulfate ratio in the waste and onthe amount of zinc recovered daily. When recovering 2000lb Zn daily from a waste with a ratio of 5 to 6, the oper-ating and maintenance costs are 12.5–14.0 cents/lb of Zn.The cost of purchased zinc oxide is 15.6 cents/lb equiva-lent zinc.

—E.G. Kominek, I.M. Abrams, S.E. Smith,E.C. Bingham, L.J. Bollyky, A.F. McClure, Jr.,R.D. Buchanan, W.C. Gardiner, G.J. Crits,L.W. Canter, R.A. Conway, D.M. Rock

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Cosgrove, J.H. Chemical recovery in viscose plants. American ViscoseDivision, FMC Corp.

Courtaulds Ltd. Fr. Pat. 2,000, 930.Crits, G.J. 1968. Boiler feedwater treatment review. Brewers Digest.

April.Gardiner, W.C., and F. Munoz. 1971. Mercury removed from waste ef-

fluent via ion exchange. Chem. Eng. 78:57. (23 Aug.).Hindin, E., and P.J. Bennett. 1969. Water reclamation by reverse osmo-

sis. Water and Sewage Works 116(2):67.Kehoe, R.A., J. Cholak, and E.J. Largent. 1944. The concentrations of

certain trace metals in drinking water. J. Amer. Water Wks. Assoc.,36:637.

Kunin, R., and R. Hetherington. 1969. A progress report on the removalof colloids from water by macroreticular ion exchange resins. 30thInternational Water Conference. Pittsburgh, Penna. (Oct.).

Kunin, R., and D.G. Downing. 1971. Ion exchange system boasts morepulling power. Chemical Engineering, 78(15):67.

Landerdale, R.A., Jr. 1951. Treatment of radioactive wastes by phos-phate precipitation. Ind. Engr. Chem. 43:1538.

Linke, W.F., Solubilities of inorganic and metal organic compounds (4thed.). Princeton, N.J.: VanNostrand.

McKee, J.E., and H.W. Wolf. 1963. Water quality criteria, 2d ed.Resources Agency of California. State Water Quality Control BoardPublication 3-A.

Mount, D.I. 1966. The effect of total hardness and pH on acute toxic-ity of zinc to fish. Air and Water Pollut. Int. J. 10:49.

National Academy of Sciences—National Research Council. 1971. Newspecifications (limits of impurities) for mercury in PotassiumHydroxide Solution and Sodium Hydroxide. Food chemicals codex,1st ed. Washington, D.C. (1 February). p. 13 and p. 16.

New Jersey Zinc Co. 1971. Chem. Eng. 78:63. (22 Feb.).O’Connor, J.T., and C.E. Renn. 1964. Soluble-adsorbed zinc equilibrium

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Chemiefasserindustrie. Wass. Abwass. Forschung. (1):7.Othmer, D.F., and A. Steinden. 1967. Encyclopedia of chemical tech-

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Pickering, Q.H. 1968. Water research. London, England: Pergamon.Rock, D.M. 1971. Hydroxide precipitation and recovery of certain metal-

lic ions from wastewaters. Water-1970. Chem. Eng. Prog. Symp.Series. 67:107. pp. 442–444.

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Boiler Blowdown WaterBlowdown is water discharged from boiler systems con-taining a relatively high concentration of suspended anddissolved solids. The discharged blowdown is replaced byfresh (usually demineralized) low-solids feedwater.Excessive solids buildup in boiler water can cause carry-over into the steam drum in the absence of suitable an-tifoam agents, or scaling when salt solubility is exceeded.Boiler blowdown water is generally alkaline and often con-tains suspended matter from sludges of insoluble sulfatesand carbonates. High temperature, dissolved solids, andalkalinity present disposal problems for untreated blow-down water, unless a larger wastewater stream is availablefor dilution before discharge into receiving water (IUPAC1963). For petrochemical plants, the blowdown stream isa minor contributor to the overall plant effluent disposalproblem (Beychok 1967).

The rate of blowdown at equilibrium must ensure thatsolids introduced into the boiler by the feedwater are to-tally removed. Since chlorides are soluble and none are in-tentionally added to the boiler feedwater, they provide ameans for measuring total salts in boiler water. If chlorideand total soluble salt concentrations in the feedwater areknown, the ratio of feedwater chloride to boiler water chlo-ride indicates the ratio of feedwater total salts to boilerwater total salts. The equilibrium total dissolved solids al-lowable in the boiler water are a direct function of the op-erating pressure of the steam-generating system (DeLorenzi 1951). A simple equation for determining the re-quired rate of blowdown is given by Equation 8.4(1)

X 5 }C1

b

002

CC

f

f

} 8.4(1)

where:

X5% blowdown (based on steam produced)or

}10

1000

1

XX

} 5 % blowdown (based on water fed)

Cf 5 total chloride concentration in the feedwaterCb 5 total chloride concentration in the boiler water

Boiler blowdown requirements are dictated by the lim-its on total allowable dissolved solids concentration inboiler water; by the economics of heat transfer for theboiler system; and by the methods of disposal or treatmentfor recovery of blowdown stream (Hamer, Jackson, andThurston 1961). Blowdown may be intermittent or con-tinuous. For intermittent blowdown, volumes are small,

dissolved solids are controlled by frequency and durationof blowdown, and settled sludge is properly removed.Continuous blowdown is preferable because it providessteady control of dissolved and suspended solids concen-tration in boiler water. It can be used if the continuousblowdown rate is above the minimum of 200 l/hr (1 gpm).Heat recovery is also easier with continuous blowdown.Intermittent blowdown, however, is still required at longerintervals to remove sludge accumulation. Antifoam agentspermit higher dissolved solids concentrations in the boiler,decreasing the rate of blowdown.

Spent Caustics from RefineriesMost refineries use caustic treatment for various productstreams to remove hydrogen sulfide, mercaptans, pheno-lic compounds, thiophenols (sulfur-bearing aromatics),and naphthenic acid impurities. Caustic treatment of cat-alytically and thermally cracked gasoline produces a spentcaustic that is rich in phenolates and thiophenolates, withlesser amounts of sulfides and mercaptides, commonly re-ferred to as phenolic caustic. Treatment of fuel gas, LPG,and straight run gasoline produces sulfidic caustic, rich insulfides and mercaptides, with only small amounts of phe-nolates. Both types of spent caustics emit a foul odor andpose serious disposal problems. If dumped into a receiv-ing stream, they will discolor water, impart an objection-able taste, exert a high chemical oxygen demand, and poi-son fish.

PHENOLIC

Several methods are utilized for recovery and disposal ofspent phenolic caustics. Table 8.4.1 briefly describes andcompares the major methods.

The best approach to phenolic caustic disposal is to re-cover the valuable cresylic acids (phenol, cresols, xylenols)and thiophenols for use as base materials in making plas-tics, wire insulation, lubricating oil additives, rubber re-claiming agents, and other products. Recovery can beachieved by neutralizing (springing) spent caustic with fluegas or mineral acids, forming an aqueous salt solution fromwhich the cresylic acids and thiophenols can be decanted.A typical flue gas neutralization system is shown in Figure8.4.1.

Both neutralization methods present obstacles. Thesodium salt solution is too concentrated for release to afreshwater stream; in addition, it normally contains 5,000


8.4INORGANIC NEUTRALIZATION AND RECOVERY

TABLE 8.4.1 DISPOSAL OR RECOVERY OF PHENOLIC SPENT CAUSTICS

Operating Information or Beneficial Possible AdverseMethoda Description Conditions Characteristics Characteristics

Sale of Phenolic Phenolic caustics are sold to a Concentrated caustic (25–30°Be9) Phenolic caustics can be sold Shipping charges may be exces-Causticsb commercial plant spe- should be used for gasoline at a profit for the refiner. A sive for some locations. Gaso-

cializing in the recovery of treatment to minimize spent commercial recovery plant is line treatment facilities mayvaluable cresylic acidsc, thio- caustic shipping charges. Phe- more capable of preventing need to be revised in somephenols, aromatic disulfides, nolic caustics should be segre- pollution problems from de- refineries before switching toand sodium salts. gated from sulfidic caustics. veloping during processing of concentrated virgin caustic.

spent caustics.Flue Gas Neutral- After contacting with flue gas Neutralization tower operates Flue gas is more economical The Na2CO3 solution is usually

izationd in a batch or continuous sys- at about 180°F and 2–5 psig. than mineral acids. Cresylic contaminated with phenols,tem, cresylic acids and thio- Flue gas and steam strip most acids and thiophenols are thiophenols and H2S. If thisphenols phase separate from of the H2S and mercaptans recovered for sale. The final solution cannot be sold, a seri-the Na2CO3 which is formed. from solution. Final pH is pH is above 8.5 and corro- ous disposal problem still ex-Sulfides and mercaptans are above 8.5. sion problems are not as severe ists. The process is subject tocarried overhead with the flue as when using mineral acids. foul odors. Burning of H2S andgas. mercaptan off gases produces

SO2 emissions.Mineral Acid After neutralization, cresylic The pH is usually reduced to Valuable cresylic acids and Low pH requires corrosion re-

Neutralizatione acids and thiophenols phase 3–4. Stripping steam is used to thiophenols are recovered for sistant construction material.separate from the sodium salt ensure maximum liberation of sale. Resulting sodium salt Reagent costs are higher.solution. Considerable H2S and H2S and mercaptans. Oper- solution may be less contam- If the contaminated salt solu-mercaptans stay in solution, ating pressure is essentially inated than with flue gas neu- tion cannot be sold, a seriousunless stripping steam is used. atmospheric. tralization. disposal problem still exists.Process can be batch or con- This process has the sametinuous. odor and SO2 problems as flue

gas neutralization.Burning Phenolic caustics can be burned Incinerator operates at 1200°– Sulfur, in the thiophenols, H2S Installation of a fluid bed in-

simultaneously with other oily 1500°F. Na2CO3, Na2SO4, and and mercaptans, reacts to form cinerator is very expensive.wastes in a fluid bed inciner- ash are withdrawn from the Na2SO4. Odors are not a prob- Flue gas must be cleaned toator. incinerator and hauled to a lem since complete combustion prevent particulate emissions.

landfill. occurs. Na2CO3 and Na2SO4 mayleach into the soil duringrainfall.

Deep Well Disposal Spent caustics are pumped into Caustics must initially be Daily operating attention is min- Groundwater contamination isa deep underground forma- tested to determine if they imized. There are no by- possible if formation is nottion with impervious are compatible with the products to be sold. tightly sealed. Plugging mayrock above and below. formation. occur in the formation if

chemical reactions occur.Biological Oxidation Small quantities of dilute caus- Aerated lagoons, activated This method is limited to small Spent caustics are toxic to bio-

tics can be mixed with con- sludge, and trickling filter quantities of dilute caustics. logical organisms at low con-taminated wastewaters and be units can be utilized. centrations. This method in-biologically degraded. creases dissolved solids in the

plant effluent and createssevere odor problems.

Ocean Dumping Spent caustics are dumped about Wastes are diluted with sea Low cost Spent caustic is toxic to some100 miles offshore in federally water or discharged so they sea creatures if concentratedapproved areas. sink. sufficiently.

aMethods are listed in descending order of overall ability to recover or dispose of phenolic caustics.bThe Merichem Company purchases phenolic caustics.cCresylic acids 5 phenols, cresol isomers, and xylenol isomers.dSome NaHCO3 is formed simultaneously with Na2CO3.eH2SO4 or HCl can be utilized.


to 10,000 (Beychok 1967) ppm phenolics and 3,000 ppmH2S. Therefore, it must be purified and concentrated intoa salable product. H2S and mercaptan off gases may cre-ate air pollution in the form of sulfur dioxide if burned.These gases, plus thiophenols, create severe odor problemsif leaks or spills occur.

These problems can be solved by large commercialplants specializing in the recovery and marketing of phe-nolic caustic products. Such firms purchase concentratedspent caustics from numerous refineries. In most cases therefiner can actually profit from the sale of the waste ma-terials (Price 1967).

In caustic treatment, a fluidized bed incinerator con-verts caustic to sodium carbonate and sodium sulfate with-out creating air pollution (EPA).

SULFIDIC

Sulfidic spent caustics present a more difficult disposalproblem. Recovered products have little or no marketvalue, especially if contaminated with undesirable materi-als. A summary of the available treatment and disposalprocesses is given in Table 8.4.2. One popular disposalmethod is to oxidize the sulfides to thiosulfate and the mer-captans to disulfides (API 1969), reducing toxicity andoxygen demand significantly. The disulfides are decantedand the thiosulfate solution is released to the plant sewersystem. However, this increases dissolved solids concen-trations in refinery effluent waters.

Steel Mill Pickle LiquorFor a metallic surface to receive a high quality protectivecoating, it must be cleaned. Steel surface cleaning usually

involves a detergent wash, a rinse and/or acid wash, fol-lowed by a rinse. In this process, called pickling, the acidwash cuts through surface oxide layers to expose brightbase metal. Pickling, continuous or batch prepares a sur-face suitable for plating, galvanizing, and other surfacetreatments. Since about 50% of integrated steel mill prod-ucts may be acid pickled, and because most plating linesutilize acid pickling, the steel industry accounts for mostacid consumption in the United States.

THE PICKLING PROCESS

Sulfuric acid was historically the acid of choice; however,hydrochloric acid has displaced it. Hydrochloric acid, al-though more expensive, pickles much faster than sulfuricacid, with less base metal loss. New automatic high-speedsteel mills require the integration of rapid systems, in-cluding pickling. Operating speeds of 600 ft/min are re-ported.

Sulfuric acid is fed to the pickling bath at a concentra-tion of about 20% and it is considered spent when halfits acid value is replaced by ferrous sulfate, FeSO4.Hydrochloric acid fed at about 20% concentration is al-most completely consumed before it is “spent.” The spentliquor is estimated as 35–45 lb of pickling acid per ton ofsteel, resulting in 8–15 gal of spent pickle liquor per tonof steel. For an annual steel production of 50 million tons,an estimated 500 million gal of spent liquor are producedannually.

The pickling area is followed by the rinsing-neutraliza-tion area. This may consist of several baths in series, withpickle liquor flowing cocurrent or countercurrent to thesteel. Pickle bath heating may be performed by steam in-jection, which causes some dilution of the liquor, or thesteel ware may be preheated. Heated tanks, maintained attemperatures as high as 200°F, tend to concentrate theliquor due to evaporation. Balancing the acid makeup anddrag-out, and returning some drag-out to the pickle tankcan control concentration.

During sulfuric acid pickling, hydrogen is released,which attacks the fresh surface and causes embrittlement.Chemicals to pacify the surface or inhibit the rate of hy-drogen attack can be used. In addition, chemicals calledaccelerators—which promote the removal of scale with-out affecting the rate of base metal attack—are utilized.The spent sulfuric acid pickle liquor, half iron and half freeacid, may also contain a variety of additional chemicals.

Hydrochloric acid does not cause hydrogen embrittle-ment and has a high intrinsic pickling rate with little basemetal attack; additives are not usually needed. Additionaladvantages of hydrochloric acid include:

• no need for scale breaking• better surface finish• faster reaction rate than sulfuric acid• prevention or reduction of overpickling due to

slower attack rate on base metal

FIG. 8.4.1 Typical flue gas neutralization system for phenoliccaustic.

TABLE 8.4.2 DISPOSAL OR RECOVERY OF SULFIDIC SPENT CAUSTICS

Operating Information or Beneficial Possible AdverseMethoda Description Conditions Characteristics Characteristics

Sale of Sulfidic Sulfidic caustics are sold to a Concentrated caustic (25°–30°Be9) Sulfidic caustics may not return Shipping costs may be prohib-Causticsb commercial plant which re- should be used in the refinery a profit for some refineries; itive for some locations. Caus-

covers sodium sulfide, sodium to minimize shipping costs however, sales should pay for tic treating facilities withinhydrosulfide and disulfides. for spent caustics. Sulfidic the transportation charges and the refinery may need to be

caustics should be segregated eliminate the need for in- revised before switching tofrom phenolic caustics. plant disposal facilities. concentrated virgin caustic.

Burning c c c c

Deep Well Disposal c c c c

Continuous A light hydrocarbon stream is Typical operating conditions in Process is simple and relatively Process is limited to absorptionRegeneration in continuous contact with the regenerator are 215°– inexpensive. of mercaptans. Any sulfides

caustic in an absorption 240°F and 1–10 psig. Mer- absorbed will not be removedtower. The spent caustic is captan off gases released during during regeneration. Conse-regenerated by contact regeneration are burned. quently, caustic eventuallywith steam in another tower. becomes spent and poses a

disposal problem.Flue Gas Neutrali- Contact with flue gas lib- Operating conditions are the Same as for phenolic caustics, Same as for phenolic caustics,

zationd erates H2S and mercaptans same as for phenolic caustics. except cresylic acids and except SO2 emissions are morewith simultaneous formation However, the quantity of H2S thiophenols are present in of a problem.of a sodium salt solution. Only and mercaptans in the over- small quantities only.small amounts of cresylic head gas is much greater. Sul-acids and thiophenols are fur recovery is necessarypresent. in most cases.

Mineral Acid Neu- Neutralization and steam strip- Operating conditions are the Resulting sodium salt solution Same as for phenolic causticstralizatione ping liberate H2S and mer- same as for phenolic caustics. may be less contaminated except SO2 emissions are more

captans with simultaneous However, the quantity of H2S than with flue gas neutraliza- of a problem.formation of a sodium salt and mercaptans in the over- tion. Cresylic acids and thio-solution. Only small amounts head gas is much greater. Sul- phenols are present in smallof cresylic acids and thio- fur recovery is necessary quantities only.phenols are present. in most cases.

Oxidation Sulfidic caustics are continuously Typical operating conditions are The process is simple and rela- Drainage of the thiosulfate solu-in contact with air and 165°–225°F and 60–85 psig. tively inexpensive. The im- tion to the sewer greatly in-steam in a packed column. Disulfides phase separate and mediate oxygen demand of the creases the dissolved solidsSulfides are oxidized to thio- are decanted. The thiosulfate sulfides is satisfied and the content of the refinery effluentsulfate and mercaptans are solution is then released to ultimate oxygen demand is re- waters. Although the ultimateoxidized to disulfides. the sewer system. duced from 2 to 1 lb of oxygen demand is reduced

oxygen per lb of sulfide. at least 50%, the remainingCOD is still very high.

Ocean Dumping c c c c

aMethods are listed in descending order of overall ability to recover or dispose of sulfidic caustics without creating additional pollution problems.bThe Merichem Company purchases sulfidic caustics.cSame as for phenolic caustics. Refer to Table 8.4.1.dSome NaHCO3 is formed simultaneously with Na2CO3.eH2SO4 and HCl can be utilized.

• lower acid content of spent acid• improvement of subsequent processes due to sol-

ubility of compounds formed during pickling• cleaner rinsing• better drying• improved overall quality of steel produced

Hydrochloric acid pickling is not without problems.The acid is more expensive than sulfuric acid, and morecorrosive. When converting from sulfuric acid to hy-drochloric acid operation, storage, pickle tanks, and fumehoods must be replaced.

DISPOSITION OF SPENT LIQUOR

Spent pickle liquor disposal methods include discharge toa waterway, hauling by a contractor, deep well disposal,recovery of acid values, neutralization, and regenerationof both acid and iron values.

Recovery of Acid Values

Recovery of acid values refers to the removal of iron saltsfrom the spent liquor by concentrating then cooling theliquor, which crystallizes the ferrous salts. The resultingmother liquor contains less iron, and its acid strength isreturned to operating levels by the addition of concentratedacid.

Ferrous sulfate crystallizes as the heptahydrate. If thecrystal is collected, dehydrated to the monohydrate, andadded in excess to the sulfuric acid spent liquor, the liquordehydrates and the heptahydrate forms and can be crys-tallized out of the cooled liquor.

Gaseous chlorine can be fed into the spent hydrochlo-ric acid liquor to oxidize ferrous iron to the ferric stateand crystallize the ferric chloride.

Recovered iron salts can be used as soil stabilizers andin water and wastewater treatment plants.

Neutralization

Neutralization is performed in an aqueous solution, form-ing an iron hydroxide that may be recovered for return tosteel melting furnaces. With neutralization, iron values arerecovered as iron oxide.

Lime, as ground limestone, calcium carbonate, or ce-ment clinker flue dust; lime slurry from acetylene manu-facture; and powdered slag from electric furnaces havebeen used to treat sulfuric acid liquors. If the liquor is neu-tralized, a mixture of gypsum (CaSO4 z 2 H2O) and ironoxide is formed, which can be separated if the pH is con-trolled. Neutralization to a pH of 0.6 to 2.0 results in theproduction of gypsum (hydrated calcium sulfate) whichprecipitates out or is centrifuged from the liquor contain-ing most of the iron. When the pH is increased to a rangeof 6 to 10, the iron is precipitated as ferrous hydroxide

and can be separated from the additional gypsum pro-duced by a hydrocyclone or differential settling. This finercrystal can then be used to seed first stage crystallization.Titration curves (Figure 8.4.2) illustrate the neutralizationcharacteristics of iron in its two oxidation states.

A substantially more compact iron precipitate is pro-duced from ferric hydroxide. Figure 8.4.3 shows the effectof iron oxidation state on chemical sludge settling rate.The solubility product of Fe(OH)2 is 1.64 3 10214,whereas that of Fe(OH)3 is 1.1 3 10236.

Ferrous iron is oxidized to ferric iron by blowing air,oxygen, and nitrous oxide (prepared from the catalytic ox-idation of ammonia) through it. This oxidation can be car-ried out on the hot spent liquor or the cooled colloidalsuspension of ferrous hydroxide, resulting in black mag-netic iron oxide (Fe3O4), ferric oxide (Fe2O3), or ferrifer-rous oxide.


FIG. 8.4.2 Effect of oxidation state of iron on neutralizationcharacteristics of waste pickle liquor as exhibited by titration withsodium hydroxide. Key: A 5 curve for mixture of ferrous sul-fate and sulfuric acid; B 5 curve for mixture of ferrous sulfateplus ferric sulfate plus sulfuric acid; C 5 curve for mixture offerric sulfate and sulfuric acid; D 5 curve for pure sulfuric acid

FIG. 8.4.3 Effect of oxidation state of iron on sludge volumeproduced upon neutralization of waste pickle liquor by sodiumhydroxide. Key: A 5 curve for 100 percent ferric iron, 0 percentferrous iron; B 5 curve for 0 percent ferric iron, 100 percent fer-rous iron; C 5 curve for 65 percent ferric iron, 35 percent fer-rous iron

Ammonia neutralization involves a single step processwherein the sulfate is recovered as ammonium sulfate crys-tal, which can be used as fertilizer.

Spent hydrochloric acid liquors can be neutralized withcalcium compounds or ammonia. Sodium hypochloride isalso used as an oxidizing agent and to raise the pH to pre-cipitate a ferric oxide.

The small pickler (using a batch, not a continuous, pick-ling process) will first concentrate spent liquor by a factorof 10; after adding lime, the heat of neutralization vapor-izes the water, drying the mass into a pourable solid. Thesteam from the initial concentration should be condensedand reused for pickling, because it can pollute if releasedto the atmosphere.

Problems associated with neutralization include oxida-tion rates from ferrous to ferric iron, settling kinetics ofthe two hydroxides and their mixtures, separation of neu-tralization products from liquors, and final disposition ofresidues obtained. Synthetic flocculants may be of value insettling the hydroxide and in vacuum filtration of the set-tled sludges. Figure 8.4.4 shows the treatment of waste-water from a chemical rinse treatment plant (Lipták 1973).

Regeneration of Acid and Iron Values

The regeneration of acid and iron values from hydrochlo-ric acid spent liquors requires either a wet process or anelevated temperature process.

In the wet process, the liquor is treated with lime, withor without oxidation, to precipitate the iron. The result-ing calcium chloride solution is treated with sulfuric acidto precipitate gypsum and to produce hydrochloric acidfor recycle to the pickle line.

The thermal processes are usually two-step. The firststep involves drying the ferrous chloride by evaporation of

free water and hydrogen chloride; it is then oxidatively de-composed to Fe2O3 or Fe3O4 with further release of thechloride (as hydrogen chloride) by the following en-dothermic reaction:

2 FeCl2 1 2 H2O 1 1/ 2 O2 ® Fe2O3 1 4 HCl 8.4(2)

In some thermal units, the two processes occur simul-taneously, while in others the processes occur in two dif-ferent areas, which are heated to different temperatures.Evaporation can be performed at 500° to 600°C. Roasters,fluidized beds, and multiple-hearth incinerators are used,producing an iron oxide that is removed as an ash fromthe vapor stream by cyclones. The water and hydrogenchloride in the vapor are condensed to produce 20% hy-drochloric acid for recycling to the pickling line. Variouseconomizers are used to cool the leaving gases and to pre-heat the air and spent liquor feed.

Miscellaneous Treatment Methods

Ion exchange processes exchange ferrous iron for hydro-gen ions. Regeneration results in a concentrated iron saltthat can be processed by one of the several methods de-scribed earlier. Electrodialysis segregates the returnableliquor from a concentrated iron salt. Ion exchange mem-branes separate the various compartments in this process.

Spent nitric, phosphoric, and hydrofluoric acid liquorsand their various mixtures can be treated by one of theseveral processes already described. Lime will removefluorides, but hydrogen fluoride can be distilled from anacidic solution at about 200°C. If silicon enters the liquor,fluorosilicic acid will also distill off with the hydrogenfluoride.

—R.A. Conway, R.G. Gantz, S.E. Smith

FIG. 8.4.4 Treatment of wastewater from a chemical rinse treatment plant. Key: pHRC 5 pH recording controller; pHIT 5 pHindicating transmitter; FRC 5 flow recording controller; FE 5 flow element; LRC 5 level recorder controller; LT 5 level transmit-


ReferencesAmerican Petroleum Institute (API). 1969. Manual on disposal of refin-

ery wastes. Volume on liquid wastes. Chapter 11, p. 6.Beychok, M.R. 1967. Aqueous wastes from petroleum and petrochemi-

cal plants. London: Wiley.De Lorenzi, O. (ed.). 1951. Combustion engineering (1st ed). New York:

Superheater, Inc.Hamer, P., J. Jackson, and E.F. Thurston. 1961. Industrial waste treat-

ment practice. London: Butterworths.

International Union of Pure and Applied Chemistry (IUPAC). 1963. Re-use of water in industry. A Contribution to the Solution.

Lipták, B.G. (ed.). 1973. Instrumentation in the Processing Industries.Chapter VI. Philadelphia, Pa.: Chilton.

Parsons, W.A. 1965. Chemical treatment of sewage and industrial wastes.National Lime Association.

Price, A.R. 1967. Turn treating costs into profit. Hydrocarbon Processing46(9): 149.

U.S. Environment Protection Agency (EPA). Fluid Bed Incineration ofPetroleum Refinery Wastes. Project No. 12050EKT. Washington,D.C.: U.S. Government Printing Office.


8.5OIL POLLUTION

Effects on Plant and Animal LifeAbout one million metric tons of oil are dumped each yearinto the sea (Audubon 1971) from shipping accidents inport alone. Such large spills affect the health of the ma-rine ecosystem. Ninety percent of atmospheric oxygen ismanufactured in the sea by phytoplankton. The sea alsofurnishes food and minerals and controls the weather.

Oil pollution also concerns those who take their livingfrom the sea. When sea animals are killed or made dan-gerous to man, fishing is curtailed. In areas of major spills,fishing and hotel industries are affected.

TOXICITY

Effects on the marine community may be immediate orlong range. Immediate effects include the killing of up to

95% of organisms in the area of a spill or leak. Oil coatssurface organisms and may also coat benthic or bottom-dwelling organisms. Wave action and currents spread oilhorizontally, mixing hydrocarbons with water. Even afterexternal manifestations are gone, oil may still be found inthe organisms. Bacteria detoxify poisons in the oil, but theyinfluence the least toxic products first, rendering the re-maining oil more poisonous than before.

Oil absorbed by marine organisms is stored in the or-ganism’s lipid pool and is not available for further degra-dation. It will only be destroyed when the animal dies. Thisoil may be passed on through the food chain as one ani-mal preys upon another, ultimately reaching man (Figure8.5.1). Oil pollution also destroys delicate sea habitat. Asplants die, currents cause erosion. An animal can never re-cover if the habitat is gone. Oil causes especially severedamage to salt marshes and estuaries.

MARINE ORGANISMS

Mammals

Few mammals actually live in the sea—whales, seals, andsea otters are the only important mammalian members ofthe marine community. Like all mammals they possessbody hair for insulation. Oil pollution causes the hair tobecome matted, reducing its effectiveness as a thermal in-sulator. The animal either freezes or succumbs to diseasesbecause of lowered resistance. Mammals are also affectedby oil through the food chain. This is especially importantin whales since they are plankton feeders. Reduction in theplankton population may cause whales to relocate or starve.

Birds

Oil mats the feathers so that the birds cannot fly or stayafloat. This matting also hampers insulation, causing manyFIG. 8.5.1 The marine food chain.

birds to freeze. The birds try to preen and ingest the oil,which can poison them. The oil can also cause blindnessif it penetrates the eyes. Birds that manage to return totheir nests in breeding season carry oil to the eggs, pre-venting them from hatching. If the bird survives the initialonslaught of the oil, it may die later because of loweredresistance or habitat damage. Only 3–5% of birds surviveoil pollution even if they receive treatment (White and Blair1971).

Fish

Much oil pollution occurs in offshore waters vital to fishproduction. In one experiment, fish larvae that came intocontact with oil hatched prematurely and died. Nutritionin fish is affected by oil in two ways, including blockingof taste receptors, and imitating the natural chemical mes-sages that alert fish to their prey. Respiration is interferedwith either by blocking the gills with oil or by effects oftrace metals and hydrocarbons on brachial cells.Emulsified oil is more harmful to fish than surface oil.

Mollusks

As members of the benthic (bottom) community, mollusksare more harmed than other marine organisms. They livein large beds on the ocean floor and have no way to es-cape the oil. In the immediate area of oil pollution, mol-lusk beds are smothered as oil is spread vertically. In onearea, long-range pollution caused mussels to become ster-ile. Mollusks tend to assimilate hydrocarbons into theirlipid pool and store them. Oysters removed to unpollutedwater do not show any reduction in unnatural hydrocar-bon levels. These stored hydrocarbons are passed throughthe food chain and can ultimately be passed to man.

Zooplankton

Zooplankton are microorganisms living in the upper sur-face of the ocean. They include the larvae of fish, lobsters,and other marine invertebrates and are the food source forthe majority of larger ocean organisms. In the immediatearea of oil pollution, plankton are killed. This has effectsbeyond the polluted area, since many fish larvae in plank-ton are migratory species. Exposure to diesel oil increasesthe death rate of exposed organisms.

Benthic Organisms

Ocean bottom dwellers cover a wide range of organisms,including crustaceans, corals, sponges, sea anemones,worms, amphipods, and mollusks. They are smothered byoil sinking to the ocean floor. Bottom dwellers are also ad-versely affected when the bottom plants can no longer holdback erosion, causing the bottom habitat to be destroyed.The amphipods of the family Ampeliscidae are highly sus-

ceptible to oil pollution. On the other hand, the annelidworm Capitella capitata is not. It was noted that after theamphipods were dead and unable to repopulate becauseof long-term oil pollution, the C capitata population wasincreasing. In one typical oil spill area, a trawl in 10 ft ofwater disclosed that 95% of bottom life was dead or dy-ing. Organisms included lobsters, crabs, snails, and clams.

PLANTS AND OIL

Algae

Algae, the counterpart of grass, plants, and trees in thewater, provides food and shelter and helps to prevent ero-sion of the sea bottom. As oil spreads, it covers the algaeand prevents photosynthesis. As the algae deteriorates, sodoes the environment. Erosion not only destroys the bot-tom habitat; it spreads the oil further. A small concentra-tion of oil causes the destruction of native algae and per-mits foreign species to take hold, lowering the quality ofthe environment.

Phytoplankton

Phytoplankton consists of microscopic algae living on theocean surface. These tiny plants manufacture 90% of theatmospheric oxygen and also form the basis of the marinefood chain. High oil concentrations will kill phytoplank-ton. Lower concentrations stop cell division and causeslower death. A 19% extract of crude oil retarded cell di-vision in Phaedactylum tricornatum after a 4-day expo-sure. It was observed that cell membranes were damagedby hydrocarbons so that the cytoplasm leaked out.

Marsh Plants

Oil washed into tidal marshes can destroy rushes, sedges,and grasses if the area is covered. Oil clogs the stomataand makes photosynthesis impossible. Poisons in the oilmay also kill marsh plants. As the plants die the marshmay erode, destroying the habitat; oil may remain in marshsediment for years. Oil trapped in the salt marsh can makeit impossible for the marsh to become reproductive again.This not only damages wildlife but causes flooding becausethe lifeless plants can no longer absorb excess water.

Table 8.5.1 summarizes the effects of oil pollution onmarine life.

Sources and PreventionOil has long been a concern in the treatment and use ofwater because it causes foaming in boilers or evaporators,adds to deposits, reduces heat transfer and flow, interfereswith gas transfer, causes taste and odor problems, and dis-rupts biological processes. Oil is a natural substance, pe-troleum (literally “rock-oil”), taken from the earth’s up-


per strata, and its by-products have many beneficial uses.Some are called oils, such as fuel oil. However, oily ma-terials are also derived from coal, animal, and vegetablematter and are made synthetically. Oil is also a paraffinor hydrocarbon, i.e., composed mainly of the elements car-bon and hydrogen.

Although no definition is complete, oily materials haveseveral common properties. They are generally liquid atroom temperature and are less dense than and usually notmiscible with water; they spot brown paper and are flam-

mable; they tend to spread on water, producing slicks; andthey are persistent and can produce troublesome emul-sions.

The analytical method for measuring the amount of oilymaterial in a substance also defines the substance. Themost useful methods involve extracting the materials intoa solvent, such as hexane, from a quantity of water. Theseparated solvent layer is evaporated, leaving residues ofoily material related to its concentration in the sample.These residues are solvent extractables. Examples of other


TABLE 8.5.1 OIL POLLUTION EFFECTS ON MARINE ORGANISMS

PossibleImmediate Area Periphery Long-Range

Organism of Spill of Spill Damage

Zooplankton Killed Increased death rate UnknownBenthic Organisms Smothered Disorder in marine Unknown; possi-

communities bility of storedhydrocarbonsbuilding up inthe food chain

Molluscs Smothered Shellfish are steri- Unknown; possi-lized; hydro- bility of storedcarbons are hydrocarbonsstored in the building up inmuscles the food chain

Fish Hatch prematurely; if in Nutrition is upset Unknown; possi-stage may also bility of storedbe killed hydrocarbons

building up inthe food chain

Birds Blindness; birds Resistance is lowered Unknown; possi-may also freeze bility of stored

hydrocarbonsbuilding up inthe food chain

Mammals May freeze Resistance is lowered Unknown; possi-bility of storedhydrocarbonsbuilding up inthe food chain

Phytoplankton Killed Retardation of Unknown; possi-cell division; bility of storeddamage to cell hydrocarbonsmembranes building up in

the food chainAlgae Killed Replacement of Unknown; possi-

native species bility of storedwith resistant hydrocarbonsforeign species building up in

the food chainHigher Plants Killed Gradual deterior- Unknown

ation of theenvironmentdue to storageof oil in thesediment

solvents are carbon tetrachloride, chloroform, benzene,and dichlorodifluoroethylene. Each solvent has somewhatdifferent results owing to differences in properties and inthe nature of the materials extracted. Non-oily matter suchas organic acids and esters may also be extracted. Phenols,resins, sulfur, and some dyes can be included. Lighter hy-drocarbons, if present, may volatilize when the solvent isevaporated.

Other methods of analysis employed are volumetric, re-fractive index, thin-layer and gas chromatography, infra-red and ultraviolet spectrophotometry (Lipták 1972).Samples for analysis must be representative of the waterunder study. The tendency of oily materials to float, stickto surfaces, and separate from water makes taking goodsamples difficult, and it is in this area that many automaticsamplers fail. Adding mixing chambers at sampling pointsin lines aids in proper sampling. Water standards that setquantitative limits on oil or oily matter should also spec-ify the method of measurement. If a standard is based onan oily appearance, i.e., evidence of a floating layer, slick,or iridescence, then the term extractables has little mean-ing.

OILY MATERIALS

Oily materials may be classified according to their prop-erties. Physically, light hydrocarbons or solvents, such askerosene or gasoline, are less viscous and more volatilethan heavy hydrocarbons like tars and residual fuel oils.Chemically, some materials are classed as aliphatic, i.e.,straight or branched chain, saturated or unsaturated. Stillothers are classed as aromatic (unsaturated and with ringstructure). They may have chemical functionality. Theirstructure may contain acid, carbonyl or other functionalgroups and have elements other than carbon, hydrogenand oxygen, such as nitrogen or sulfur.

Oily materials can be classified as to use, such as fuels,lubricants, coatings, cleaners, solvents, cutting and rollingfluids, hydraulic fluids, carriers, and cooking fats and oils.Many uses are both domestic and industrial. Light hy-drocarbons and solvents are found in industrial wastestreams, as a result of degreasing or extraction, cleaning,painting, and coating operations. Their vapors representpotential fire and explosion hazards, and their presencemakes the removal of heavier oily materials more difficult.The pollution potential of all hydrocarbons is moderatelyhigh; however light hydrocarbons are more readily oxi-dized biologically than heavier fuels, tars, and residues.They are also a potential source of air pollution.

Industrial Sources

One of the principal industrial sources of oily wastes is thepetroleum industry. Oily wastes result from producing, re-fining, storing, or transporting operations, or in the use ofthis industry’s products. Another major oil source is the

metals industry. Most oily wastes result from metalwork-ing or forming operations. Oily materials lubricate andcool instruments or the metals being worked. Emulsifiedoily materials and finely divided suspended solids makethese wastes difficult to handle.

Coke plants generate much oily wastewater, most ofwhich is derived from cooling, quenching coke, or scrub-bing gases. Much of the contaminated water is reused orconsumed elsewhere in the mill. The wastewater containsphenols, cresol, and related extractable materials. Phenolic-type extractable materials may also be found in water usedto cool or wash cupola stack gases at foundries. In pro-cessing meat, fish and poultry, oily wastes are producedfrom cleaning, slaughtering, and processing by-products.A major source of oily wastes is the rendering process.Cooking plant tissues, seeds, grains, and nuts aids in ex-tracting their oils for commercial purposes; cooking andextraction processes result in oily wastewaters.

Most oily wastes in the textile industry result fromscouring fibers in an early process step, especially scour-ing wool. The waste liquor yields valuable lanolin, but thewastewater is also high in extractables and difficult toprocess.

Oily wastes in the transportation industry result fromleaks, spills, or cleaning operations. Tankers, barges, andtank trucks transporting oily materials must be cleaned toprevent possible product contamination. The cleaning so-lutions contain oily materials and create pollution if dis-charged without treatment. Latex in wastewaters may beextractable. Latex in the rubber industry is generally re-moved from waste streams with little difficulty. However,in the paint industry the presence of solvents, resins, andemulsifiers can make removal very difficult.

Some oily materials are introduced into water systemsduring heating or cooling steps. Oily materials may be de-rived from leaks in seals, condensers, or heat exchangersfrom the process side of the equipment. When steam usedfor direct heating of fatty or oily products, the recoveredcondensate will likely be contaminated. Run-off from in-dustrial areas following storms may be contaminated withoily materials. The rain washes processing units, walk-ways, buildings, and surrounding grounds, carrying awayoily materials deposited there.

Municipal Sources

The major sources of oily wastes are from food prepara-tion, garbage disposal, and cleaning. Cleaning includeslaundry, car washing, and general cleaning jobs where wa-ter is the main solvent and carrier. Grease and oily mate-rials are removed at sewage treatment plants. Road oil anddegraded asphalt are washed from roads into storm sew-ers and streams. Rainwater also contains soot and varioushydrocarbons washed from the faces of buildings and otherstructures in communities.


Natural Sources

Coniferous trees and shrubs contribute oily materials torun-off water, particularly in areas wooded with pine trees.

DETECTION, IDENTIFICATION, ANDSURVEYS

A survey of potential sources of oily materials is initiatedwith an inventory of known oily products or by-productsused or produced in the area. Processes, machinery, andstorage areas are checked for leaks, drips, and potentialfor spills or accidental contamination. Both cooling andcondensate-return water are examined. Stack gases aretested for hydrocarbons and soot. Cleaning and wash-down procedures are observed; run-off water is tested andmethods of handling the water noted; and potential prob-lems from startup and shutdown of processes or equip-ment are probed. Field personnel should be familiar witheach process, system, unit, and individual piece of equip-ment. In certain cases material balances are helpful.

Oil-soluble dyes help to check for leaks in complex sys-tems or to locate oil-water interfaces. Ultraviolet light de-tects as little as 0.02 ppm of some oils by fluorescence inthe dark. Dip sticks and sonic probes are useful to checkfor multiple layers in tanks and sumps. When an oily ma-terial is isolated, its source can often be identified by in-frared analysis or gas chromatography. Analysis of tracemetal content may also help indicate its source. Many oilymaterials are identified by their characteristic odor. Moreand more oily materials will be tagged1 for easier identifi-cation.

PREVENTION

Early process control can reduce the quantity of treatablewaste. Segregation as a part of early control can simplifytreatment processes. The presence of emulsifiers, wettingagents, soaps, deflocculants, and dispersants, as well asfinely divided suspended solids, makes separation of oilymaterials and the treatment of wastes more difficult.Advantages can be realized from high temperatures andlow pH levels, and the presence of substances that makenecessary pH adjustments impractical may be avoided.

Control at the source may reduce raw material andproduct losses. Oily materials recovered downstream in atreatment plant are usually more contaminated and requiremore costly refining for reuse. The ability of one processover another to produce less oily or more easily handledwastes must be considered in initial planning, along withplant site selection, availability of suitable raw materials,labor, product markets, utilities, transportation, and wa-

ter supply. Questions concerning waste treatment meth-ods and facilities must be thoroughly examined. Plant lay-out must be the best compromise between efficient pro-duction, storage of materials, and segregation, collection,and treatment of wastes.

In selecting plant equipment, pollution potential shouldbe considered in addition to cost, performance, and ser-vice life. Raw materials, processing aids, and cleanersshould be selected to simplify oily waste treatment prob-lems. Cleaners containing oils or solvents that might endup in waste streams should be avoided. Materials that formdifficult-to-break emulsions should be avoided. Impuritiesin raw materials can add to oily waste problems, yieldingbottoms, tailings, or unusable by-products from produc-tion. Poor quality raw materials may lead to off-spec fin-ished products, which must be wasted, blended off, or soldat lower profit. If an oily waste or used solvent cannot bereused, it may be taken by a jobber for re-refining or bya scavenger. Finally, it may be burned, perhaps recoveringits heating value. In general, wastes containing high con-centrations of oily materials should not be discharged tolagoons or through deep well disposal.

Often oily materials recovered early in a process can bereused following a simple cleanup step such as filtration.At other times it may be necessary to reconstitute the oilyproduct by reblending certain components depleted in useor during re-refining. This service is also offered by somesuppliers or jobbers.

One of the most important factors in preventing or re-ducing oily wastes is housekeeping and maintenance. Goodpractices are largely a matter of adequate planning, train-ing, and followup. Employees need to know the acceptedpractices in handling each oily waste. Regular inspectionsof operations must be made. Procedures should be re-viewed on a regular basis for possible revision. A goodpreventative maintenance program will do much to pre-vent accidental losses. Safe procedures for special emer-gencies should be developed, particularly for handling sud-den releases of large quantities of oily materials. Properplant design and layout helps, but specialized equipmentmay be required and employees must know how to use it.

Continuing Needs

Improved recovery and re-refining methods for oily mate-rials should increase their reuse, and attention should begiven to developing useful by-products from oily wastes.The food industry already uses some waste fats and oilsin animal feeds, and the pulping industry recovers usefultall oil which was once wasted with black liquor.

Improved methods of cleaning tanks and vessels aregreatly needed. More efficient use of dry cleaning tech-niques should be employed. Cleaning solutions should bekept segregated and renovated or fortified for reuse wher-ever possible. The dry-cleaning industry has learned thisin the reuse of its cleaning fluids. Disposable tank liners


1. A tag substance is any material not normally found in oils, which willstay with the oily phase and be detectable in low concentrations. Suchsubstances can be dyes, radioactive materials or, if there are no othersources of lead in the process, lead.

or inserts should find use in some instances; ultrasoniccleaning techniques might help reduce the volume of clean-ing wastes.

Finally, there is a continuing need for simple, rapidmethods of detection and identification of oily materials.Some progress is being made with continuous ultravioletand infrared detectors and by the addition of tag materi-als.

Methods of ControlMETHODS OF OIL SPILL CONTROL

a) Mechanical Containmenta1) floating booms, a2) bubble barriers and a3) current barriers

b) Mechanical Recoveryb1) weirs and suction devices, b11) floating weirs, b12) suctionheads, b13) free vortex, b2) lifting surfaces, b21) rotating discs,b22) rotating drums and b23) moving belts

c) Application Agentsc1) dispersants, c2) sinking agents, c3) collecting agents, c4) herd-ing agents, c5) burning agents and c6) biodegradation

Listing of Oil Sinking AgentsSee Table 8.5.2

CHARACTERISTICS AND COMPOSITION

Spreading Rate of Oil Spills

Several factors need to be vectorially combined to definethe oil spill spreading pattern. The current will drift theunrestrained oil at about the same velocity as the water.Wind adds a component of about 3–4% of the wind ve-locity, and natural spreading acts concentrically to dispersethe slick. This is initially caused by the oil’s hydrostatichead balanced by the oil’s inertia. Typically, for an im-pulsive 500–2,500,000 gal spill of 0.9 specific gravity (SG)oil, this acts for about 1/4–1 hr until the oil reaches about1/4 in thickness and a 500–3,000 ft diameter. At this point,pressure spreading is primarily balanced by viscous dragin the underlying water, and the slick diameter grows atabout 300 ft/hr. As the gravity head decreases, the net sur-face tension spreading pressure (water to air-oil to air-oilto water), usually about 20 dynes per centimeter, contin-ues to disperse the oil until typically a 0.01–0.001 in thick-ness is reached.

Variations in spreading rate depend on the oil’s specificgravity, surface tension, characteristic evaporation, solu-bility in water, emulsification of water into the oil, andpour point. In a confined area, natural surface active agentsin the oil can spread into a monomolecular film holdingup to Af in of even low specific gravity oil.

Specific Gravity

Oil spill specific gravities range from 0.75 to 1.03. Thelower values represent highly refined products such as gas-

oline, kerosene, and diesel fuels. The upper values repre-sent residual oils. Crude oils have specific gravities between0.8 and 1.0; however, this increases rapidly when the lightends evaporate. Also, with a low sea state, crudes and oilscontaining asphaltines readily form water-in-oil emulsions,raising pollutant specific gravity from 0.85 to 0.95 in sev-eral days.

An oil spill’s buoyant hydrostatic head is inversely pro-portional to the difference between the water and oil spe-cific gravities. The lower specific gravity oils spreadrapidly, but once captured by a containment boom theirbuoyancy resists entrainment into a current stream pass-ing under the oil and boom. Also, specific gravity can limitremoval of the oil from a contained pool. In this case, re-covery is proportional to the recovery device frontal lengthand the gravity-inertial feedrate per unit length (Q).

Q 5 }2H

3o

} !}2¤g¤D

3¤ H¤o}¤ 8.5(1)

where:

Ho 5 Oil Depth,g 5 gravitational constant,D 5 water SG 2 oil SG

Thus, between the specific gravities of 0.75 and 1.0 the re-covery feedrate will vary thirty-fold for a given thickness.Conversely almost a tenfold increase in thickness is re-quired to have the same effect on recovery feedrate.

Viscosity

Spill viscosities range from 0.7 to over 20,000 centistokes(cst). Residual oils, weathered emulsions and high pourpoint crudes can even reach a semisolid state. The emul-sions strongly deviate from Newtonian characteristics, fre-quently exhibiting very high viscosities at low shear ratesand much lower viscosity at higher shear rates, such asthose generated in transfer pumps. Viscosities for crudesweathered for up to a day and emulsified by moderate seasare between 300 and 1000 cst. There is no direct rela-tionship between viscosity and specific gravity. Howeverthey tend to increase together (Figure 8.5.2).

Emulsification

Water-in-oil emulsions are unstable and difficult to formin highly refined oils. However, most crudes and all resid-ual oils contain asphaltines, resins, cresols, phenols, or-ganic acids, metallic salts, and other surface-active agentsthat concentrate at the interface between entrained waterdroplets and the oil. A crude spill can become a 40% wa-ter emulsion in a single day due to open sea action. In 5days, this can increase to 80%. Increased shearing ratesand action decreases water droplet size and increases emul-sion stability. Pumping emulsions with free water may re-


sult in up to 98% water in the oil emulsions which are soformed.

Flammability

Crudes frequently contain 30–70% gasoline and benzine,presenting a significant fire and explosion hazard. Theseconditions are rapidly mitigated by evaporation and masstransport when the wind is blowing, due to their large sur-face area. Residual oils and weathered emulsions presentonly a minimal fire hazard.

MECHANICAL CONTAINMENT

Floating Containment Barriers (Booms)

Desirable characteristics for containment booms includelow cost; compact storage; easy and rapid deployment;durability; easy cleanup; and performance compatible withthe environment. Boom construction materials must beprotected from extended immersion in water or oil. Rela-tively compatible plastic materials include polyethelene,polyurethane, polyvinylchloride, polypropylene, epoxies,polyesters, nylon, and neoprene.

Even in calm, still water, the boom’s draft and free-board must be adequate and balanced to account for theoil’s buoyant head. Likewise, although there is a pressurebalance across the boom at the bottom of the containedslick, the boom must be strong enough to hold the hy-drostatic head differential above that point.

D 5 0.5 rv2ACD 8.5(2)

M 5 0.5 rv2dCDX 8.5(3)

The flat plate drag, D, of a containment boom’s immersedarea, A, normal to a current of density, r, and velocity, v,is given by equation 8.5(2), where CD is the drag coeffi-cient and D equals the drag tension loads that result. Test

measurements indicate that CD can exceed 3 for a 2–ftdraft boom filled with 0.9 SG oil in a 1–kt (knot) current.The boom’s rolling moment about its lower edge is givenby equation 8.5(3), where X is the distance to the centerof pressure, and X/d # 0.5 without oil and # 0.7 whenfilled with 0.9 SG oil. This moment must be counteractedby ballast and/or roll flotation. A large number of com-mercially available booms have the requisite strength andstability to operate in up to a 2–kt current in calm water.

CURRENT ACTION

If the boom is held at its ends, upstream from the oil spill,it will assume a catenary shape and capture the oil thatdrifts against it. The oil will form a stagnant pool with itsmaximum depth near the boom. If critical depth is ex-ceeded at the boom, there may be a rapid drainage fail-ure, during which most of the captured oil is lost. In a 1–ktcurrent, with oils of 0.8, 0.9 and 0.98 specific gravity, testsindicate that boom depth must exceed 3, 10, and 55 in,respectively, to prevent drainage. In a 2–kt current the min-imum depth almost triples. From near the boom to nearthe upstream edge, oil thickness decreases parabolically.At the leading edge, there is a rapid thickening due to agravity or head wave.

When the current exceeds 1/3 kt, oil can be torn off thebottom of the head wave and entrained in the current flow-ing under the captured pool. If the distance from the bar-rier, the barrier’s draft, and the oil’s buoyancy are insuf-ficient, the entrained oil will sweep under the boom andwill not resurface to coalesce with the captured pool. Fora 2–ft draft boom and for oil specific gravities of 0.9 and0.98, the respective critical current speeds are 2 and 0.9kts. If the current is higher and there is sufficient sea room,the boom must drift with the current to keep the relativespeed below critical. In a tidal channel or river, the boomcan be streamed at an angle such that the current velocitycomponent normal to the boom is below the critical speed.In this way, oil can be diverted without major loss frommidchannel to a lower current area or to the river bank.

WAVE ACTION

Waves cause cross-coupled surge, heave, and roll of theboom. If wave surge and current tension loads are takenthrough an independent bridle, they will have a minimumtendency to constrain the boom’s heave response. Mini-mizing boom mass tends to increase the natural heave fre-quency and minimize the lag in heave response. Thus, bal-lasting for stability or even adding weight for flotation canbe critical to heave. Flotation placement away from thefaces of the boom yields stability and decreased lag in heavemotion. If natural frequency exceeds wave frequency, theboom will follow the wave and minimize surge forces.

If the boom does not readily heave or surge, the localforce will be very high. Also, as the free water line mean-ders from the boom’s still waterline, there are changes inroll movement. This is accentuated by current and wave


FIG. 8.5.2 Oil specific gravity vs viscosity at 60°F.

surge. Thus, such a heavy-duty, slow sea response boommust be exceedingly strong, rugged and have a generousfreeboard and draft. An example is the Merrit-Navy boom,consisting of 10–ft sections, each with 3–ft draft and 3–ftfreeboard. It is constructed of 4–ft by 8–ft by 3/4 –in ma-rine plywood with a 100–lb ballasted fabric skirt. Four oildrums strapped on for flotation add to the section weight.All of this requires two 1/2–in wire ropes to be used for thetension bridle.

An independent bridle takes the tension loads out of anumber of independent boom segments. Each segment canbe connected to the next by a slack skirt. Lines trailingfrom the bridle to the boom segment permit the best heaveresponse but provide no roll resistance. Multiple lines fromthe bridle to the top and bottom of each segment can limitroll at the sacrifice of heave. Such a design was developedby Johns-Manville for the U.S. Coast Guard. This boomis compliant and responsive to sea action. It has survived15–ft waves with swells breaking on the barrier.

A number of booms have been employed on open oceanspills but there are no quantitative reports of their perfor-mance as a function of sea state and current. The U.S.Coast Guard boom is of minimum weight and is highlytransportable (Hoult et al. 1970). Their 1972 tests in openseas of over State 32 were the first full-scale instrumentedevaluations conducted. At up to 1–kt current, the oil leak-age rate was very low. In these tests it held over 25,000gal of oil in a thickness of 21/2 in.

Bubble and Current Barriers

A surface current generated by spray nozzles or by the up-welling from a bubble stream can oppose oil spreadingpressures. The simple bubble barrier consists of a sub-merged pipe with numerous air discharge holes along itslength. Its primary use is to encircle fuel or oil loading ter-minals, or to protect ship berths. However, it cannot beused if the natural current is over the 1/4 -kt normal to thebarrier. In this case, the turbulent countercurrent causedby the bubbles will form a head wave and carry the oildown into the main current, which then passes the oilthrough the bubble stream. When the bubbles emerge fromthe pipe, they expand to a maximum size of 1/4- in diame-ter as they rise. The bubbles will break up before becom-ing much larger. Thus, the air pressure required needs onlyto exceed the pipe’s hydrostatic pressure. The number ofholes in the pipe and their size is not critical. The risingbubble column creates an upward flow of water that up-wells and then generates a surface current. Up to 4-in thicklayers of oil with SG under 0.92 have been held under sta-tic conditions by this technique. The maximum surface

current (Jones 1970) is given in equation 8.5(4), where Qis the air flow in ftm per ft of barrier and Vm is in fps. ForSG # 0.9, the oil thickness held (Jones 1970), h, in inchesis given by equation 8.5(5),

Vm 5 1.47 (gQ)1/3 8.5(4)

h 5 0.6 Q0.45/1 2 SG 8.5(5)

MECHANICAL RECOVERY

Weirs and Suction Devices

A conventional weir with a smooth receding entrance, op-erating in a sufficiently deep contained oil pool, will per-form in accordance with the governing rate depicted byequation 8.5(6) (Figure 8.5.3). The recovery rate/ft for oneedge of a weir in a static pool of oil is plotted as a func-tion of the weir immersion depth. The plot of the spread-ing rate/ft for typical oils shows how deep the oil must beto support a given removal rate. In this case the oil mustbe more than twice as deep as the weir immersion depth.If it is not, oil flow will be broken and the weir will floodwith underlying water. This explains the importance ofsetting these devices to avoid water flooding. It also showsthat thin layers of oil cannot be recovered without largeamounts of excess water. The same principle applies tosuction devices.

In a current, weir flowrate over the lip increases in ac-cordance with the rate equation 8.5(6). However, the flow-pipe below the weir lip can act as a barrier (Figure 8.5.4).

Q 5 }23

} CLw √2wgw 31Hw 1 }V2g

20}2

3/2

1}V2g

20

}23/2

4 8.5(6)

FIG. 8.5.3 Weir flow and gravity inertial flow vs oil depth.Key: Q 5 volume rate of flow over weir, in ft3/sec; Lw 5 lengthof weir, in ft; C 5 discharge coefficient < 0.6 (light oil withbeveled, inward shaped weir); g 5 acceleration of gravity, in ft/sqsec; Hw 5 head, undisturbed level about weir, in ft; Vo 5 ve-locity of advance of weir, in ft/sec.

2. From Sea State Table by Wilbur Marks, David Taylor Model Basin:small waves, becoming larger; fairly frequent white horses; moderatebreeze (14 to 16 kts); 4–ft significant wave height (5–ft average of aQ;thhighest); 65–ft average wave length; 28-naut. mi minimum fetch; 5.2 hrminimum duration of wind.

If care is not taken, oil accumulates in front and drainagefailure occurs. This becomes relevant when wave surge ispresent. Failure can be caused by a vortex at the weir endsthrough which the whole pool of oil drains. If current isnot present and surge is reasonably low, small waves in-crease the average rate of flow due to 3/2 power depen-dence on head. However, care must be taken to avoidflooding. The usual practice is to provide a large flotationresponse area to match heave to wave motion and to pro-vide for surge motion to avoid drainage conditions.

To overcome weir difficulty of operating in thin slicks,various techniques of increasing oil depth may be used.The inverted weir, consisting of a shallow shaped barrierbacked up by a second barrier, is one method used. In op-eration, thin slicks are diverted under the first barrier, andthe current entraining the oil is recirculated to the surfaceas part of a vertical and countercurrent flow caused by thesecond barrier. In a more refined and improved design,secondary currents are avoided and oil resurfacing for con-centration in the protected basin occurs due to the oil’sbuoyancy. This still requires a conventional weir or suc-tion device to provide final oil removal from the thickenedpool between the barriers.

Many weirs are custom designed. The U.S. Coast Guarddeveloped a prototype for open sea use consisting of abasin formed by two parallel booms joined by a flexiblebottom. Oil and water flow over the first boom or throughslots in it. The water gravity separates from the oil and es-capes through slots in the flexible bottom. A sonic sensordetects the oil level in the basin, activating weir removalof the oil by the second boom.

Lifting Surfaces

This class of recovery devices depends on moving anoleophilic, hydrophobic surface through the floating oillayer. The motion creates a viscous shear and attaches anoil boundary layer to the surface. The surface is then with-drawn above the waterline, where the oil is extracted andpumped to storage. The advantage of this device over theweir is that it will not flood with water when the oil isthin or the immersion depth varies.

One lifting surface device, the vertical disc, can be effi-ciently packaged into a small volume with disc spacing of2 in or less. The device was originally available fromCentri-Spray Corp. for calm water applications. An im-proved version called “Clean Sweep” using discs and vaneswas commercially introduced by Lockheed (Bruch andMaxwell 1971). Model test data indicated that the mod-

ification will function at over 2 kts current or forward wayin as high as Sea State 43 and will recover all oil encoun-tered to a thickness of at least 0.1 in. These tests were con-ducted with disc diameters from 10 in to 8 ft.

In a 4-ft diameter it will recover 100 GPM of 1000 cstoil per ft of device in a 1-kt current. It can recover oilswith over 10,000 cst viscosity. The U.S. Coast Guardawarded Lockheed a contract to develop a prototypeair-transportable system for oil recovery use in up to SeaState 4.

Moving belt units were successfully used to clean upweed and peat moss contaminated residual oil from a spillin cold Canadian waters. A small pond type unit is alsoavailable. Similar to the moving belt is the rotating drum.The drum is usually employed only in calm water appli-cations. One type, developed by Amoco and the subjectof several improvement versions, uses an absorbentpolyurethane foam-covered drum. Different density foamcovers are used for various viscosity oils to the maximumrecommended 2500 cst.

APPLICATION OF AGENTS

Dispersants

Many agents will disperse oil in water; generally, their useis prohibited by the EPA unless they protect human life,property from fire, or an endangered species from moredirect damage. Both biological toxicity information andoxygen requirements for biodegradation are necessary toobtain an intelligent appraisal of their safety.

Sinking Agents

Sinking agents are used only with approval in waters morethan 100 meters deep, where there are no fisheries or on-shore currents. Approval is only given when no other fea-sible means of control is available. They present a signifi-cant logistical problem, since a pound of agent will onlysink 1/2–5 pounds of oil. An efficient agent should be per-manently hydrophobic, permanently oleophilic; high indensity; inexpensive; readily available; and easy to spread.Table 8.5.2 compares the characteristics of a number ofcommercially available materials, of which only stearicacid-treated chalk has been used in quantity.

Collecting Agents

The two major types of collecting agents are gels and ab-sorbents. Although several types of gels exist, they aremoderately expensive and difficult to apply, and there is


FIG. 8.5.4 Weir drainage failure in current.

3. Moderate waves that take a pronounced long form, many white horses;Fresh Breeze (18 to 19 kts); 6.5 ft. significant wave height (8.3 ft. aver-age of 1/10 th highest); 95 ft. average wave length; 55 nautical mile min-imum fetch and an 8.3 hour minimum duration of wind.

no existing equipment suitable for recovery. The most fre-quently applied absorbents are hay and straw, because theyare inexpensive, widely available, and easy to apply withmulching equipment. However, their application to float-ing oil requires slow trash bucket and rake recovery sinceall of the mechanical, weir, and suction-type recovery de-vices are clogged by the straw or their recovery rate is dras-tically reduced. Their best use is for cleaning up oil onbeaches or in tidal pools. Straw will absorb about fivetimes its weight in oil.

Polyurethane foams have the largest oil sorption ca-pacity. They can hold 30–80 times their own weight in oil.Urea formaldehyde foam is equally effective. Polyethylenefibers and shredded polystyrene foam are only about halfas effective. However, a patented grafted expanded poly-styrene is reportedly as effective as polyurethane. Of thenatural products, wood cellulose fiber and shredded red-wood fiber are one-third to one-fifth as efficient aspolyurethane. Ground corncobs are about as effective asstraw. Foams take more time to absorb higher viscosityoils. On the open sea, wind prevents efficient distributionof the expensive low density absorbents. The more ex-pensive materials also require that squeezing out the oiland reprocessing be economical.

Herding Agents

A novel method of oil recovery involves magnetism to sep-arate the oil from the water and to lift it. A ferrofluid, con-sisting of a stable colloidal dispersion of super paramag-

netic particles, is mixed with the oil. However, there is noapparent advantage over mechanical methods where sur-face tension wetting and fluid shear attachment are suffi-cient for both separation of oil from the water and re-covery. The latter approach avoids all expense for bothtreatment agent and its application.

Another method is the use of a nontoxic, biodegrad-able surface active agent to surround an oil spill and tochange significantly the water-air surface tension. The U.S.Naval Research Laboratory reported on the use of sorbi-tan esters of fatty acids and polyoxyethylene alkyl ethersas monomolecular surface films having 40 dyne per cen-timeter spreading pressure. Also, Shell Oil Co. commer-cially introduced a similar acting proprietary chemicalcalled “Oil Herder.” The recommended application rateis 20 gal/mi of spill perimeter.

Burning Agents

Because of the rapid loss of volatile substances from an oilspill, ignition and the support of combustion requires as-sistance. Despite the cost implication and other problems,burning is attractive because it provides rapid disposal ofa large quantity of oil with a minimum of material han-dling. Most burning agents are nontoxic or inert. The ma-jor problem is distributing the agent and the need for abetter ignition technique. Among several agents evaluatedby the EPA are straw, cellular glass beads and silane-treated fumed silica. All of the agents appeared capable ofsupporting combustion, even in extremely cold waters, by


TABLE 8.5.2 TYPICAL OIL-SINKING AGENTS

Materials withRelatively PermanentOil Retention lb Oil per Handling ApplicationCharacteristics Pound Agent SG Availability Hazards Method

Barite treated 1.3 3.0 Limited, None known Drywith latex need sprinkle

treatment onlyfacility

Chalk treated 1.5 2.7 5 ton; None known Drywith stearic acid NYC; sprinkle

Europe & agitateAsbestos (treated 4; must agitate 2.4 Unlimited; Asbestos is Spray in

100% hydrophobic) to sink with time minimum if solutionfor treatment distributed in of water

solution toavoid accidentaldust inhalation

Carbonized treated sand 0.4 2.6 Limited; need Possible silicosisFly ash, 0.5 2.7 treatment

chlorosilane or facility Drysilane treated sprinkleand neutralized only

Sand, 120 g silane treated 2.7and neutralized

acting as a wicking agent. Based on very limited testing,the beads gave the best performance, followed by straw,and finally the fumed silica. Wind creates a severe prob-lem in applying the agents, including the silica, which wasmixed with water. Wind and waves also break up uncon-tained pools and cut off continued combustion. Both thebeads and the fumed silica, when not used in combustion,appear recoverable by normal mechanical recovery sys-tems. The straw presents a special problem.

Biodegradation

There are many toxic products in oil, most of which areassociated with the lighter volatile compounds. Many mi-croorganisms can utilize hydrocarbons as an energy sourceto convert them into cell mass. On a pilot plant basis, anoil company has produced protein from residual paraffinbase oil by bacterial fermentation. One source estimatesthat consumption rates will be limited eventually by the2–lb oxygen demand to convert 1 lb of oil into cell mass.They further estimate that under moderate conditions thereare about 25,000 lb of oxygen absorbed per sq mi of seaper day. Hence, normal oxygenation may limit oil con-sumption to 12,500 lb/sq mi/day.

There may have to be an addition of nitrogen and phos-phorus in the form of ammonium nitrate and potassium

phosphate. Industrial waste systems normally consume 20lb of nitrogen and 40 lb phosphorus with this much oxy-gen, whereas the sea normally has less than half thisamount available. Experiments by the Department ofOceanography, Florida State University, showed that se-lected microbial cultures can accelerate the removal ofparaffinic crudes at a rate twice that of evaporation. Therate almost doubled with a 10°C rise in temperature; sur-factants were produced, hastening emulsification.

—L.S. Savage, R.S. Robertson, B. Bruch

ReferencesAudubon. 1971. Oil pollution. Vol. 73:3 (May). p. 101.Bruch, B., and K.R. Maxwell. 1971. Lockheed oil spill recovery device.

Joint Conference on Prevention and Control of Oil Spills. Washing-ton, D.C. (June 15–17).

Hoult, D.P., R.H. Cross, J.H. Milgram, E.G. Pollak, and H.J. Reynolds.1970. Concept development of a prototype lightweight oil contain-ment system. U.S. Coast Guard Rpt. No. 714102/A/003. (June).

Jones, W.T. 1970. Air barriers as oil spill containment devices. SPE 3050,45th Annual Fall Meeting of the Society of Petroleum Engineers.Houston, Tex. (Oct. 4–7).

White, P.T., and J.S. Blair. 1971. Bare-handed battle to cleanse the bay.National Geographic 139. (June). p. 877.


8.6PURIFICATION OF SALT WATER

Conversion ProcessesThree principal methods of augmenting the world’s sup-ply of potable water are:

1. Cloud seeding to furnish artificial production of rain2. Advanced waste treatment to render wastewaters di-

rectly reusable3. Conversion of salt water by a variety of processes

The first has been used with some success in arid areas,but the results are not always predictable. The second haspassed from a laboratory procedure to the construction offull-size plants which are treating water in certain SouthAfrican communities and returning it directly to the dis-tribution systems to deliver drinking water to the popula-tion.

Conversion of salt water to fresh water is not a newidea. It is, in fact, the oldest and most extensive processknown. Each day the power of the sun evaporates millionsof tons of water from the oceans, which returns to the

earth as rain. Desalted water has chemical and physicalproperties similar to rainwater. It is low in total solids,corrosive, clear, generally odorless, and somewhat insipid.It may contain dissolved gases, and water desalted by themembrane process may still contain a major part of traceelements (such as boron) originally in salt or brackish wa-ter.

The great attraction of desalting water is that over 97%of the world’s supply of water is saline; furthermore, muchof it is contiguous to arid regions. An example of the im-portance of desalting is the experience at Kuwait. Duringthe early 1950s, the Kuwait Oil Company built an oil re-finery dependent on desalted water from the Gulf. In 1953,the Kuwait government put into service a then-large ca-pacity (1.2 mgd) desalting plant of 10 triple-effect, sub-merged tube evaporators. This installation is also an ex-ample of economics favorable to desalting, as the heatsource for the evaporative process was waste or naturalgas.

Oceans. The oceans and seas contain 97% of theworld’s 326 million cu mi of water. One cu mi is equalto one trillion gal. Sea water contains about 35,000 ppmof salts.

Salt Lakes. In a few places there are lakes or seas withno outlet. The resultant removal of water by evaporationhas left behind a liquid in which the salt content is nearsaturation. Examples are the Dead Sea in Israel and theGreat Salt Lake of Utah. It is reported that when water islow in the Great Salt Lake, the water contains 250,000ppm (25%) of common salt.

Ground High-Salinity. Some groundwater in the westernUnited States and in the vicinity of oil fields may be clas-sified as brine for it contains upward of 10,000 ppm salt.

Surface-Brackish Water. Some surface streams and estu-aries contain salt of 2000 to 5000 ppm and are thereforeunsuitable to drink. They may be ideal as sources of de-salted water because of the low initial salt content.

Three basic types of desalting processes include evapo-rative, membrane, and freezing, and each of these has sub-types or alternative methods.

Evaporative (Distillation) Processes

The four subtypes in this group are multistage flash (MSF),submerged tube (ST), long tube vertical (VTE), and vaporcompression (VC). The MSF process contains three flow-streams, namely influent seawater, recycled brine andproduct effluent water. Cold seawater is pumped througha heat exchanger where the heat gained by the seawateris furnished in part as the heat loss of the condensing prod-uct water. Partly heated seawater then goes through an at-mospheric degassing tank and joins the brine in the firstevaporative stage. The brine is pumped to the second stage,passing through condensers around which more productwater is formed by condensation of fresh water evaporatedfrom concentrated brine in the second stage. Condensedproduct water from both stages accumulates in trays andflows to a product water sump, from which it is pumpedto points of use.

The vertical-tube evaporator (VTE) system is somewhatsimpler. It consists of vertical tubes installed in heat ex-changers arranged in series. Seawater falls through thetubes in the first section where it absorbs heat and con-denses fresh water.

The submerged tube (ST) process operates somewhatlike a conventional boiler. Typified by small capacity dis-tillation equipment on ocean-going vessels, the VC processoperates as a vertical tube evaporator either with boilingbrine inside the tubes or flashing above the tubes. It owesits name to the fact that the fresh-water steam is pressur-ized.

Electrodialysis and Reverse Osmosis

Electrodialysis and reverse osmosis both use membranes.In the electrodialysis process salt water passes between lay-ers of membranes that are selectively permeable or imper-meable to ions in the salt water, depending on the mem-brane charge. Brine is produced in one part of the deviceand fresh water in the other, and the energy of separationcomes from an electric current.

The heart of the reverse osmosis process is a semiper-meable membrane separating salt and fresh water. Thenormal phenomenon of the flow of fresh water throughthe membrane to the salt water side is reversed by apply-ing pressure to the salt water side.

Freezing

If saline waters are cooled sufficiently, they freeze, and theresulting ice is fresh water. The ice crystals are separatedfrom the brine, cleansed and melted.

Desalination PlantsData on desalting plants by type of process are given inTable 8.6.1. The selection of any one or combination ofprocesses can be justified by economic and other factorsas listed below:

1. The salt content of the source2. The salt concentration acceptable in the desalted plant

effluent3. Logistics, i.e., the location of the proposed plant, its

size, the labor market, and alternative sources of water4. Available sources and costs of energy5. The costs of alternative methods of supplying water6. Provisions for the disposal of brine: is there a local salt

market?7. How much fresh water is required

Salt Content of Plant Influent andEffluent

Where there is a choice of influent concentration, the se-lection of the water with the lowest salt content reducesenergy requirements and costs. The most abundant sourceof salt water is the ocean, which on average contains about35,000 ppm salt. Utilizing the sea and converting it to freshwater provides water supplies to islands in the Aegean Seaand in the South Pacific, where solar stills are used.1 Theseare of the greenhouse type, requiring a rather extensivearea.


1. Transparent plastic is supported on a framework arranged so that asthe vapors rise from the pool of salt water under the canopy, they con-dense on the underside of the plastic and run into drains or troughs toa storage point.

The U.S. Public Health Service Drinking WaterStandards recommend a limit of 250 ppm for chloride and500 ppm for total dissolved solids. When water is pro-duced by desalination in areas governed by these stan-dards, this level of salt and dissolved solids is specified asan upper limit. The International Standards for drinkingwater set the maximum allowable total dissolved solidslimit at 1500 ppm.

Plant Location and Energy Source

The location and construction of a desalting plant ofteninvolves transporting construction materials for great dis-tances. It is most economical to build a plant on the edgeof the sea, providing large amounts of energy to convertthe salt water to fresh water and pump the fresh water in-land. The cost of fuel is a critical factor in the choice ofany distillation process, and the cost of electric power isequally important in electrodialysis processes.

An inevitable by-product of a desalting plant is brine(concentrated salt water). When a plant is located at theedge of the sea, the brine is emptied into the ocean itself.The point of discharge must be chosen to minimize ad-verse effects upon the ecology through temperature eleva-tion and salinity at the point of outfall. In some areas ofthe world, there is a market for the salt produced by ad-ditional evaporation of the brine.

The requirements for fresh water determine both thetype of plant and the choice of fuel. For example, on anocean-going vessel the demand for fresh water may be only20,000 gal/day. There are places in Australia where solar

distillation is used to produce as little as 600 to 1200gal/day. At the other end of the spectrum are the waterneeds of large cities, which exceed millions of gal/day.

Desalting ProcessesOptimization of energy utilization is one of the principalfactors in the design of a desalting process. Energy—eitherthermal, mechanical, nuclear or electrical—is required. Thecost of energy may represent as much as 50% of the to-tal water production cost. The percent of recovery of freshwater from salt water, a basic design criterion, rests onconsiderations of energy utilization. Starting with seawa-ter having a concentration of 35,000 ppm of sodium chlo-ride, the minimum isothermal work required at 70°F toseparate 1000 gal of seawater is 2.6 kw-hr. However, thisassumes removing water from an infinitely large volumeof seawater, or pumping an infinite amount of raw sea-water through the plant with little recovery of fresh wa-ter. Theoretical work for separation increases with in-creasing recovery (recovery is the ratio of product to feedexpressed as a percentage), while pumping cost decreaseswith increasing recovery. The total energy required as afunction of percent recovery is expected to follow the curve(Gilliland 1955) shown in Figure 8.6.1.

The optimum level of recovery for seawater is about30–50% or about 2–3 volumes of feed to 1 volume ofproduct water. For brackish water the optimum is about3–8 volumes of feed to 1 volume of product. The higherpercent recovery for brackish water feed is desirable be-cause it is available in limited quantities compared to sea-water, and also because disposal of concentrated brine isusually a problem.

The principal factors in the design of an evaporationsystem (Othmer 1970; Gilliland 1955; Rubin 1963) are


TABLE 8.6.1 DESALTING PLANTS BY TYPE OFPROCESSa

Total PlantType of Process Number of CapacityUsed Plants (MGD)

Evaporationmultistage flash (MSF) 229 146.3long tube vertical (VTE) 96 54.4submerged tube (ST) 302 38.7vapor compression (VC) 19b 2.2b

646 241.6

Membraneelectrodialysis (ED) 34 5.1reverse osmosis (RO) 3 0.2

37 5.3

Freezingvacuum freezing 3 0.3vapor compression (VC) 3 0.3Totals 686 247.2

aAdapted from Office of Saline Water Special Report on Status of Desalting,November 1970.

bDoes not include 3000 vapor compression units on vessels sailing theoceans; each of these can produce several thousand gal per day.

FIG. 8.6.1 Work requirements for the separation of seawateras a function of recovery. Product per feed in percent.

conservation of energy, separation of evolved vapors frombrine, heat transfer, and prevention of fouling of the heat-ing surfaces. Of these, conservation of energy is para-mount. The cost of water would be prohibitive if steamwas used on a once-through basis, as in a single-effect evap-orator (Figure 8.6.2).

Figures 8.6.2 and 8.6.3 show evaporation systems thatpermit reuse of latent heat of vaporization of the primesteam supplied by the boiler. In multieffect evaporation(Figure 8.6.2), water evaporates at the highest pressure inthe first effect. This vapor is condensed in the second ef-fect to evaporate an approximately equal amount of va-por. Proceeding in this manner, the amount of steam andcooling water used per lb of fresh water produced is re-duced.

In vapor compression evaporation (Figure 8.6.2), thewater vapor evolved is superheated. This vapor is com-pressed to a pressure or saturation temperature sufficientto provide the necessary heat transfer. The condensed wa-ter vapor is the product water, and the residue brine isused to preheat the incoming salt water.

In multistage flash evaporation (MSF) (Figure 8.6.3),the operating principle is described. Multiple-effect andMSF operations not only reduce the amount of steam re-quired and the cost of the boiler, they also save coolingwater; hence the capital cost of the condenser and coolingwater supply system is also reduced. Vapor compressionoperation eliminates condenser cooling water and almosteliminates the need for a steam boiler.

MULTIEFFECT EVAPORATION

Figure 8.6.4 is the flowsheet of the 1 million gpd 12-effectevaporator OSW Demonstration Unit (Guccione 1962) in-stalled at Freeport, Texas. In this plant, seawater feed isheated to about 130–135°F. To prevent deposits on evap-orator heat transfer surfaces, sulfuric acid is added to lowerthe feed pH and to react with the scale-former:

CaCO3 1 2H1 ® CO2 1 H2O 1 Ca21 8.6(1)

Mg(OH)2 1 2H1 ® Mg21 1 2 H2O 8.6(2)

Gases in the seawater and carbon dioxide formed arestripped in the deaerator. To prevent corrosion, pH is thenadjusted to about 8 by adding caustic. This treated wateris further preheated, charged to the top of the first verti-cal tube evaporator (VTE), passing through a distributorplate into long tubes (extreme right of Figure 8.6.4). Steaminjected midway on the shell side of the tube bundle iscondensed, heating the seawater. The seawater feed flowsto the lower section of the vaporizer where the vapor isdisengaged. The generated vapors enter the shell side ofthe next effect (to the left) and condense to give fresh wa-ter. Brine from the first effect becomes the feed to the topof the second effect. The remaining effects operate in thesame manner at progressively lower temperatures andpressures. Finally, fresh water from each effect is pipedthrough a heat exchange system for preheating incomingseawater.

With 12 effects and some 20 heat exchangers, about 10lb of fresh water is obtained per lb of prime steam used.By operating at temperatures no higher than 261°F and ata concentration factor below 3, the scale problem is con-

FIG. 8.6.2 Evaporators. A. Single-effect; B. Four-effect; C.Vapor compressor

FIG. 8.6.3 Multistage flash evaporation. After being heated tothe highest temperature in the brine heater at the top of the di-agram, the seawater passes while flash evaporating and coolingat successively lower pressures (vertical arrows down on left side).The vapors leaving each flash evaporation (horizontal arrows)pass to preheat the seawater in the condensing-heating tubes(wide vertical arrow on right side). Freshwater condensate alsopasses from higher to lower stages for evaporation and cooling,to discharge at the bottom. Vapors may also be withdrawn fromthe brine heater to be condensed in the half-stage (lines upperright) to increase production of fresh water.

trolled. The production rate of this plant exceeded designcapacity.

Among the main drawbacks of a vertical tube evapo-ration system such as the one described are: a large num-ber of metallic tubular surfaces are required for evapora-tion and condensation; and many heat exchanger surfacesare needed for recovery of heat from the streams of saltwater and fresh water. These are all expensive.

Two developments may remove these drawbacks. Oneis the development of a high-performance fluted tube bythe Oak Ridge National Laboratory for film evaporation

in VTE and for condensation. Increasing effectiveness ofthe tube’s heat-transfer surface may reduce the cost of theVTE system. The other development is an MSF system torecover the heat. This VTE-MSF process may reduce thecost of heat-recovery surface and obtain additional waterin the MSF section (Browning 1970).

VAPOR COMPRESSION EVAPORATION

Figure 8.6.5 is the flowsheet (Browning 1970) of the 1 mil-lion gpd OSW Demonstration Plant at Roswell, New


FIG. 8.6.4 Multieffect evaporation. Flowsheet of 1 million GPD OSW demonstration plant at Freeport, Texas.

FIG. 8.6.5 Forced circulation vapor compression plant. Flowsheet of 1 million GPD OSW demonstration plant at Roswell, NewMexico.

Mexico. Alkaline brackish water from an artesian well isthe feed. This water contains 15,000 ppm of mixed saltand is richer than seawater in scale formers, especially cal-cium salts. For scale prevention the water is first fedthrough an ion-exchange system, removing about 87% ofcalcium. It is then preheated to 145°F and treated withsulfuric acid for reaction of the remaining calcium andmagnesium salts. Gases such as carbon dioxide, nitrogen,and oxygen are removed in a vacuum degasifier; caustic isthen added to neutralize the pH.

The treated water is pumped to 130 psia, heated to214°F and fed to the first-effect through the suction sideof the brine circulation pump. In the forced circulationevaporator, the saline water is heated by the compressedsecond-stage steam, which condenses to fresh water. Thesaline water flashes at the top of the tube at the rate of 1lb of steam per 250 lb of water circulated. The vapor fromthe dome of the first effect heats the saline water in thesecond effect and condenses to give fresh water. The va-por from the dome of the second effect is compressed toheat the first effect. The compressor operating across thetwo effects is a five-stage axial-flow machine driven by a2,000-hp electric motor.

The high circulation rate of water in the forced circu-lation evaporator is intended to obtain a heat transfer co-efficient as high as possible. Thus, the necessary heat trans-fer is satisfied without excessively large surface areas, whilethe required temperature difference is minimized. The re-lationship between this temperature difference and thepower consumption by the compressor and the circulationpump has an impact on plant and water costs (Othmer1969; Gilliland 1955).

The sludge recycle system (Figure 8.6.5) serves as abackup for the ion-exchange system for scale prevention.This system is devised to achieve a 1% slurry in the firsteffect so that scaling material will precipitate on the seedcrystals and not on the heating surfaces. As compared tothe multi-effect evaporation plants, the VC plant occupiesless space and uses mainly electrical energy. The energyused is about 60 kwh per 1,000 gal of fresh water pro-duced. For these and other reasons, the vapor compres-sion unit is applicable where demand is small and a com-pact, efficient unit is required. It is used to supply potablewater on ships and in areas where inexpensive power isavailable.

The force-circulation evaporator with its high velocityof circulation makes this process useful for processingbrackish water containing large amounts of scale-formingsalts. High reliability of mechanical components, such asthe compressor, is essential. Also, the practical size of thiscompressor limits the size of the vapor compressor evap-orator. The high performance heat-transfer surfaces de-veloped for the VTE system may also be helpful. Likewise,the MSF process may recover heat from brine blowdownin large-scale operations.

MULTIFLASH EVAPORATORS

In a 2.5 million gpd plant designed for OSW as a stan-dard or Universal Desalination Plant (Othmer 1970)(Figure 8.6.6), seawater at 85°F is first sent to the heat re-jection stages (Figure 8.6.3) where it is heated to 97.7°F.Part of this seawater is discharged, while the other part istreated for scale, corrosion, and foam prevention. The


FIG. 8.6.6 Universal desalination plant (2.5 million GPD).

aSJAE = steam jet air exhaustbMSF = multistage flash

treated seawater joins the discharge from the heat rejectstages and goes to the lowest temperature stage of the heatrecovery system. It passes through all the stages and isheated to 250°F in the brine heater with prime steam fromthe boilers. The hot seawater returns to the MSF where itevaporates and cools at successively lower pressures, firstin the recovery stages, then in the heat reject stages. Theexit brine stream is split, part for recycle and part for blow-down. The vapor released in flashing in each stage con-denses to heat the incoming seawater and gives fresh wa-ter.

The MSF plants now supply 97% of fresh water de-rived from seawater. In a single-effect MSF (Figure 8.6.6)with 40–50 stages, as high as 8–11 lb of fresh water areproduced per lb of prime steam used. With a multirecy-cle, three-effect MSF at Chula Vista, California, the gainratio increased (Othmer 1970) from 8 to 11 to 20.

Scale formation on heating surfaces, a troublesomeproblem in the two preceding evaporation processes, isminimized here. Evaporation takes place by flashing at suc-cessively lower pressure, therefore no heat transfer surfaceis needed for evaporation. However, precipitation of salts,such as calcium sulfate scale in the tubes in high temper-ature parts of the system, is still a problem. The solubilityof these salts in water decreases with increasing tempera-ture.

For heat conservation, equilibrium between the coolerseawater from each stage and the vapor formed withoutsalt entrainment is desired for this process. In practice itis not possible to obtain this in the present MST (Othmer1970). Another drawback is the use of large numbers offlash stages, usually forty or more. The large number ofstages is required to reduce individual temperature dropor flash temperature and to reduce violence of ebullition(boiling), which causes entrainment.

Variations in MSF design for increasing the gain ratioor lb of fresh water/lb of steam used, without addition inplant cost, are possible (Othmer 1969). Controlled FlashEvaporation (Roe and Othmer 1971) (CFE) would allowequilibrium between vapor and liquid throughout thestages and higher flashing temperature ranges withouttransport losses of pressure and temperature and salt en-trainment.

Freezing ProcessesFreezing processes involve cooling incoming seawater,freezing it to obtain freshwater ice, separating the ice andbrine liquor, melting the ice to give fresh water, and usingthe purified water and concentrated brine to chill the in-coming seawater.

Freezing has several advantages over evaporation, mostimportant that latent heat of water fusion is only aboutone-seventh the latent heat of evaporation—thus freezingprocesses hold the promise of low-energy power require-ment; and low temperature operation minimizes the main-

tenance associated with scale formation and corrosion.However, freezing processes have some inherent disad-vantages, including the fact that freezing time is longerthan that for vaporization of water; and cleaning the saltfrom the ice and handling the ice crystals is quite difficult.

VACUUM-FREEZE VAPORCOMPRESSION

Figure 8.6.7 shows the Zarchin-Colt process in which wa-ter is the refrigerant. Seawater, after prechilling in a heatexchanger to a temperature approaching the freezing pointof brine, enters the freezer. Evaporation is induced by thesuction of the compressor, which absorbs heat from theremaining brine. Ice crystals are formed, growing to about0.5 mm. The ice-brine slurry is pumped to the ice decanterwhere ice crystals float to the top. There they are washedwith a portion of the product water made. This washwa-ter moves downward and carries away salt. A rotatingscraper trims the ice layer at the top and sends the washedcrystals to the freezer, where they come into contact withthe compressed water vapors. The water vapor condenseson the ice crystals suspended on a rotating perforated trayand melts them. (The ammonia refrigeration system shownin Figure 8.6.7 removes heat gain through insulation.) Thismelted ice is the desired fresh-water product. The cold freshwater and cold brine from the ice decanter are routedthrough a heat exchange unit to chill the incoming sea-water. This process was successfully operated in a 100,000gpd pilot plant. The total power cost is estimated, for animproved operation, to be as low as 27.3 kwh per 1,000gal in a combination of 21/2 million gpd units.

One drawback of this process is the large vapor vol-ume prevailing under vacuum (about 1/200 th atm) in thefreezer. This requires a large diameter compressor, which


FIG. 8.6.7 Zarchin-Colt freezing process for seawater desalt-ing.

limits the size of a single unit to about 200,000 gpd. Largercapacity plants must consist of duplicate units. To cir-cumvent this, a process was developed involving a sec-ondary refrigerant. Instead of water vapor, a refrigerantsuch as butane, which has a vapor pressure above atmos-pheric at the freezing temperature of brine, is used. Liquidbutane is mixed with the brine in the freezer and, after ab-sorbing the heat of ice fusion, it vaporizes. After com-pression, the butane vapor is used to melt the ice crystals.However, this development work was terminated becauseprojected costs for commercial plants were no lower thancosts for evaporation.

Reverse OsmosisIn this process, water from a salt solution is forced acrossa selectively permeable membrane by a pressure difference.The membranes allow water to pass through but not saltions. The pressure applied must be greater than osmoticpressure. The osmotic pressure in a freshwater-salt watersystem separated by a selectively permeable membrane isa direct function of salt concentration: about 25 atm forsea water and 1.4 atm for brackish water with 2,000 ppmsolids.

Figure 8.6.8 shows a typical reverse osmosis system.Salt water is first pumped through a filter to remove grossparticles and iron, and then it is subjected to additionalpretreatment as required to prevent fouling of the mem-brane surface. Some examples of brackish water pretreat-ment include lime-soda treatment or the addition of se-questering agents to prevent calcium precipitation. The saltwater is then pressurized to a level high enough to reversenormal osmotic pressure and to provide driving forceacross the system, including the membrane. The salt wa-ter is fed into reverse osmosis cells (modules). Part of thewater permeates the membrane and is collected as freshwater. The remainder (brine) passes through a turbine forrecovery of power before it is rejected.

Figure 8.6.9 shows a spiral-wound reverse osmosismodule. The module contains spacers, modified celluloseacetate membrane, and a porous backing in a spirallywrapped double sandwich. Brine flows across the mem-brane sheets while the product water flows toward the cen-ter of the wrap and out the unit’s core. Another promis-ing membrane configuration involves the use of fine hollowfibers.

A 50,000 gpd reverse osmosis unit using spiral woundmodules was installed and successfully operated at RiverValley Golf Course, San Diego, California. Feedwater witha concentration of 4,500 ppm dissolved solids was takenfrom an unused well. The unit was operated at 600 psigwith a recovery of 75% (3 gal product water for every 4gal feed) and yielded a product with a salinity of less than350 ppm. Thus, reverse osmosis should be considered forconverting brackish water to potable water. The future ofreverse osmosis processes for large-scale installations

hinges on the reduction of membrane replacement costs,a major item in total water cost. The objective is either re-ducing basic membrane cost or extending service life. Alarge part of the present work on reverse osmosis is di-rected toward increasing product-water flux while main-taining salt properties (Browning 1970).

ElectrodialysisElectrodialysis is based on the development of membranesthat are selective for the passage of ions of a given charge.Two different membranes are used: one is more selectiveto anions, and the other is more selective to cations. Electriccurrent aids the diffusion of these ions, and the electric en-ergy required is proportional to the concentration of saltsin the saline water. Therefore, the process is more attrac-tive for desalting of brackish (low salt concentration) wa-ters.

Figure 8.6.10 shows a multicellular arrangement for de-salting brackish water. The unit consists of alternative an-ion-permeable and cation-permeable membranes, and saltsolution is passed through all compartments. The adjacentcells have the anion-permeable and cation-permeablemembranes on the opposite side. The imposition of an elec-tric potential causes the cations to migrate to the cathode,and the anions to the anode. However, neither cations noranions, after passing from the feedcells into the adjacentbrine cells, can pass through more than one cell towardthe electrodes because they are blocked by impermeable


FIG. 8.6.8 Reverse osmosis for seawater desalting.

FIG. 8.6.9 Spiral membrane module for a reverse osmosis unit.

membranes. Thus, the feedstream is depleted while the ad-jacent stream is enriched in ions. The plant produces apotable water stream and a salt-rich stream which is re-jected.

Ion-exchange resins, which comprise 60–70% of themembrane, are solidly hydrated, strong electrolytes andmight be regarded as solid sulfuric acid or as caustic solid.The resin most commonly used is polystyrene cross-linkedwith divinylbenzene. The ion-exchange resin permeable tocations is made by sulfonating the polystyrene resins; theresin permeable to anions contains a quaternary ammo-nium group attached to the polystyrene resin.

Electrodialysis is an established process for desaltingbrackish water. Units with capacities from 10,000 to650,000 gpd have been installed. The process has majoradvantages for brackish water but is considered too costlyin electric power requirements for desalting seawater.

Keeping the membrane surface clean is a major prob-lem. Prefiltering and chemical treatment of feed has keptplants operating, but a thorough study of brine composi-tion should be made and special pretreatment methodsshould be developed as required. Membrane replacementcosts are a major part of producing fresh water by thismethod. Current research includes studies to improve se-lectivity of the anionic-permeable and cationic-permeable

membranes to increase the maximum allowable currentper unit membrane area and to develop feedwater pre-treatment processes for the removal of various membrane-blocking contaminants.

Table 8.6.2 summarizes the most favorable feed prop-erties and capacities for each of the six processes describedin this section.

Processes based on evaporation are operated in large-scale plants for desalting seawater. Out of the several evap-orative processes, MSF has been most widely used in verylarge-scale installations, while VC is commonly used insmall-scale plants. Vapor compression with forced circu-lation evaporators offers the possibility for desalting brack-ish water.

Processes based on semi-permeable membranes are usedmainly for desalting brackish water. Electrodialysis is al-ready a well-established process, although reverse osmosisis also feasible. Controlled flash evaporation appears to bean attractive alternative to the MSF process.

The Future of DesalinationThe most successful evaporative desalting method is theMSF process (Figures 8.6.3 and 8.6.6). These units arebuilt with as many as 69 stages and produce up to 20 lbof water/lb of steam. Research continues to improve theefficiency of this process by flash enhancers and by com-bining the MSF system with multiple-effect evaporationprocess steps.

Problems common to all desalting processes include thedecreasing but still high combined cost of equipment andoperation (see Conversion Processes) and the low load fac-tors associated with these plants. The load factor refers tothe percentage of time that the plant is in operation, andthis seldom exceeds 60% for desalting plants. Low loadfactors are partially caused by corrosion problems and par-tially by scaling problems. For seawater, scaling becomesa problem at 160°F, although methods have been devel-oped to control scaling at temperatures as high as 350°F.In addition to maintenance problems, scaling also degradesthe heat transfer efficiency of heat exchange surfaces.


FIG. 8.6.10 Electrodialysis used to remove salt from brackishwater.

TABLE 8.6.2 SELECTION AND APPLICABILITY OF DESALTING PROCESS

Type of Feed PlantDesalting Favorable Concentrationa CapacityProcess Feedwater Range (ppm) Range (MGD)

Multistage flash Cold-soft 5,000–35,000 .1Verticle tube evaporator Cold-soft 5,000–35,000 .5Vapor compression Warm-soft 5,000–35,000 1–20Vacuum-freeze vapor-

compression Cold-hard 5,000–35,000 0.25–5Reverse osmosis Warm-soft 3,000–10,000 0.1–10Electrodialysis Warm-soft 1,000–4,000 0.1–10

aDissolved salts

In membrane-type desalting processes, the research hasbeen directed toward both reducing membrane cost andincreasing membrane life span.

Today there are over 1000 desalination plants in oper-ation, converting over a billion gallons of saltwater perday into potable water.

—F.B. Taylor, D.H.F. Liu, C.J. Santhanam

ReferencesAnon. 1971. Why hollow-fiber reverse osmosis won the top CE price for

DuPont. Chem. Eng. 78: 54. (29 November).Brennan, P.J. 1963. Fresh water from vapor-compression evaporation.

Chem. Eng. 70: 170. (14 October).Browning, J.E. 1970. Zero in on desalting. Chem. Eng. 77: 64. (23

March).Ellwood, P. 1970. Chem. Eng. (2 Nov.) pp. 46–48.Gilliland, E.R. 1955. Fresh water for the future. Ind. Eng. Chem. 47(12):

2410.

Guccione, E. 1962. Old method for fresh-water needs. Chem. Eng. 69:102 (26 November).

Johnson, J.S. 1966. Hyperfiltration. In Spiegel, K.S. (ed.), Principles ofdesalination. New York, N.Y.: Academic.

Larson, T.J. 1970. Reverse osmosis pilot plant operation: a spiral mod-ule concept. Desalination, 7: 187.

Merten, U. 1966. Desalination by reverse osmosis. Cambridge, Mass.:MIT Press.

Othmer, D.F. 1969. Evaporation for desalination. Desalination, 6: 13.Othmer, D.F. 1970. Kirk-Othmer: Encyclopedia of chemical technology.

2d ed. Vol. 22, New York, N.Y.: Interscience.Rickles, R.N. 1966. Membrane: Technology and economics. Park Ridge,

N.J.: Noyes Development Corporation.Roe, R.C., and D.F. Othmer. 1971. Controlled flash evaporation. Chem.

Eng. Prog. 67(7): 77.Rubin, F.L. 1963. Perry’s Chemical Engineers’ Handbook. 4th ed. New

York, N.Y.: McGraw-Hill.Weismantel, G.E. 1968. Recycle boost desalting efficiency. Chem. Eng.

75: 86 (15 July).Wilson, J.R. (ed.). 1960. Dimineralization by electrodialysis. London:

Butterworth.


8.7RADIOACTIVE LIQUID WASTE TREATMENT

Radioactive liquid wastes usually contain high, medium,or low amounts of radioactivity. The means of treatinghigh-volume–low-activity waste is very different from themeans of treating small-volume–high-activity waste.Medium-level waste is treated to convert it into high- andlow-activity waste. Low-activity wastes are treated to re-move radioactivity, then discharged to the environment inamounts well below permissible limits. This is the diluteand disperse philosophy. High-activity wastes, because oftheir hazard, must be concentrated, contained, and re-

moved from man’s environment. Definitions of waste cat-egories (ASI 1967; IAEA 1970a) are given in Table 8.7.1.

Low-Activity WastesPRECIPITATION

Low-activity radioactive wastes are collected and mixedfor a more uniform effluent or segregated for specific treat-ment of individual components. In the first approach,

TABLE 8.7.1 RADIOACTIVE WASTE DEFINITIONS

IAEAa Common

Activity LevelASAa

Activity LevelCategory A(mCl/ml) Category Activity Level Category (Cl/l)

1 A , 1026 A A , MPCpb Low ,1026

2 1026 , A , 1023 B MPCp , A , MPC0b

Intermediate ,1023

3 1023 , A , 1021 C MPC0 , A , 104 MPC0

4 1021 , A , 104 D 104 MPC0 , A , 108 MPC0

High ,15 104 , A E 108 MPC0 , A

aIAEA, International Atomic Energy Agency; ASA, United States of America Standards Institute.bMPCp, maximum permissible concentration for members of the population at large; MPC0, maximum permissible concentration for 40-hr work week occupational

exposure.

wastewater flocculation, precipitation, sorption, filtration,and ion exchange can be adapted to radioactive wastes.Typical removals (Straub 1964) of mixed fission productsand individual nuclides are shown in Table 8.7.2.Common methods for mixed fission product precipitationare aluminum salts, iron salts, tannic acid with lime, phos-phate with lime, ferrocyanides, and excess lime-soda ash.

When wastes are segregated, specific treatments for ra-dioactive isotopes include strontium, combined calcium,nonradioactive strontium-iron phosphate, or hydroxide atpH 11.5, tannic acid, and nickel ferrocyanide; cesium,nickel ferrocyanides, nickel ferrocyanides, and copper andiron ferrocyanides; and ruthenium, nickel, copper, or ironferrocyanide. Processes are chosen based on local consid-erations to obtain a high degree of radioactivity removalat a high floc settling rate, with a minimum sludge volumeand a maximum degree of economy.

Decontamination factors of 10 may be obtained formixed fission products and decontamination factors of 200for specific isotopes with a specific treatment. Provisionmust be made for discharging decanted water and for dry-ing, packaging, and storing sludge from coagulation andprecipitation. Residues are presently sent to an approvedcommercial burial site.

ION EXCHANGE

If the total solids content of the wastes is low (less than1000 ppm), the volume of waste is small, or a final pol-ishing of effluents is necessary, ion exchange is a suitabletreatment method. At commercial power plants, ion ex-change, filtration, and evaporation are the major processesused for liquid radioactive waste treatment. Both sulphonicand phenolic-carboxylic resins are used. Typical ion ex-change units for waste treatment at boiling water reactorpower plants include one 200 gpm mixed bed with no re-generation; one 75 gpm mixed bed, with no regeneration;and one 50 gpm mixed bed with no regeneration. Someboiling water reactors use very fine, 90% less than 325mesh, ion exchangers as filters and ion exchange beds withno regeneration (Goldman 1968).

At pressurized water reactor power stations, typical ionexchange treatments include one 12 gpm mixed bed unit,no ion exchange in waste disposal system; 4 mixed bedunits; and 45 ft.3 cation exchangers (Goldman 1968). Allof the reactors use ion exchangers in coolant purificationoperations. On June 7, 1971, the Atomic EnergyCommission published a schematic diagram of the generalconcept of radioactive waste handling systems for light wa-ter-cooled nuclear power reactors (Figure 8.7.1).


TABLE 8.7.2 TREATMENT PROCESSES FOR REMOVAL OF RADIOACTIVE WASTES

Decontamination Factorsa

Individual Mixed FissionProcess Radionuclides Productsb

ConventionalCoagulation and settling 11.0–1001 1.12–9.1Clay addition, coagulation and settling 11.0–100 11.1–6.2Sand filtration 11.1–100Coagulation, settling and filtration 11.1–50 11.4–13.3Lime-soda ash softening 11.2–100Ion exchange, cation 11.1–500 12.0–6.1Ion exchange, anion 11.0–125Ion exchange, mixed bed 1.11–3300 .150–100Solids-contact clarifier 11.9–15 12.0–6.1Evaporation 1.00–10,000

NonconventionalPhosphate 11.2–1000 .125–250Metallic dusts 11.1–1000 11.1–8.6Clay treatment 11.0–1001Diatomaceous earth 11.1–`Sedimentation ,1.05Activated sludge 1.03–8.2 14.8–9.8Trickling filter 1.05–37 13.5–6.1Sand filter 18.3–100 11.9–50Oxidation ponds ,1.1–20

aDecontamination factor 5

bNo data listed implies lack of information, not unsuitability of the process.

initial concentration}}}final concentration

EVAPORATORS

Evaporators can obtain high decontamination factors, 104

to 106, if carryover is eliminated or if the evaporator is fol-lowed by ion exchange of condensate.

The evaporator at the National Reactor Testing Stationis typical of evaporators used at research and productionsites. The continuous evaporator is constructed of stain-less steel (Type 347) and is a thermosyphon type with anexternal heat exchanger of 48 sq m area, rated at 800,000kcal/hr heat duty. Vapor passes to an entrainment cham-ber with 4 bubble cap trays, where it is scrubbed withclean water. The evaporator is capable of processing 1800l/hr. The tritium in the wastes is not concentrated at all,but the other radionuclides are concentrated by a factorof 50, and the condensate is decontaminated (Lohse,Rhodes and Wheeler 1970) by a factor of 2000.

DILUTION AND RELEASE

All processes require some decontaminated waste to be re-leased into the ground, local waterways, or the atmos-phere, or that waste is recycled. No plants completely re-cycle wastes, although some newer reactors will do so.Studies at the Oak Ridge National Laboratory show thatit is possible to treat low-activity waste by zeta-potentialcontrolled additions of alum and activated silica to removecolloids. This is followed by demineralization and decon-tamination by cation and anion exchange resins, the pas-sage through a column of activated carbon to removecobalt and organic materials (Blanco 1966). The residues,exhausted resins, and activated carbon, must be packagedand sent to a storage facility.

Ground disposal of decontaminated liquid waste tookplace at many sites, but the practice lost favor because ofthe uncontrolled nature of the release and the irreversibil-ity of the process (IAEA 1967). The process takes advan-tage of the slow movement of groundwater (enablingshorter half-lived radioisotopes to decay), the ion exchangeproperties of soil, and if above the water table, the capacityof the unsaturated soil to store moisture. In addition, thereis slow dispersion of wastes in the groundwater system.

HYDROFRACTURE

One method of low and intermediate concentrate disposalto the ground that maintains control of wastes is hy-drofracture. In this method, a well is drilled to the desiredgeological formation and cased. Then, the casing is perfo-rated at the specific depth desired, pressure is applied, andthe formation is fractured. After the formation is fractured,a radioactive waste mixture containing portland cement,fly ash, attapulgite, illite, delta gluconolactone, and trib-utyl phosphate is injected into the space and spread as athin sheet parallel to the bedding (Figure 8.7.2). The ra-dioactive waste forms a dense solid with improved cesiumretention on the illite and improved strontium retentionon the fly ash. The delta gluconolactone retards set times,and the attapulgite is a suspender, reducing the quantityof cement required.

This technique was first used in December 1966, whenthe first ultimate disposal of radioactive waste took placewith the injection of 72,000 gal of intermediate waste con-taining 20,000 curies of cesium-137 in a shale formation870 ft below the ground surface. The method was alsoroutinely used at Oak Ridge National Laboratory and wasdemonstrated at the Nuclear Fuel Services site.

Hydrofracture techniques are used where there are thickformations of shale in flat-lying, well-bedded, sedimentaryrock for intermediate activity wastes and possibly for lowand high activity wastes with suitable modifications.


FIG. 8.7.1 Nuclear power reactor for liquid waste handlingsystem.

FIG. 8.7.2 Disposal of intermediate activity wastes by hy-drofracture.

BITUMINIZATION

Although not used in the United States because of the ad-equacy of present techniques and the high costs of con-version to new methodologies, bituminization is favoredfor installations in other countries for solidification andimmobilization of low and intermediate activity sludgesand residues (IAEA 1970b). The process is easy, cheap andnot dependent on waste type or storage location. Wastesare introduced into asphalt or emulsified asphalt and thewater is removed. Radiation levels of 108 rads (see glos-sary) do not cause soft asphalts to swell significantly, nordo they increase the leach rate.

High-Activity WastesGENERATION

High-activity wastes are generated when irradiated fuel el-ements are reprocessed to recover unfissioned uranium andto remove the fission products with large neutron ab-sorption cross-sections.

Spent fuel elements are cooled for 150 days or more,allowing shorter-lived fission products, particularly I131, todecay. The end pieces of the fuel element are cut off andthe main element sheared into small pieces and leachedwith hot nitric acid to dissolve the UO2. The leached hullsare rinsed and sealed in 30-gal drums and buried as solidwaste. The radiation level is 10,000 R/hr. The nitric acid-uranium-fission product impurities solution then goesthrough the Purex solvent extraction process to recoverand decontaminate uranium and plutonium from the het-erogeneous solution. The solvent is tributyl phosphate dis-solved in n-dodecane, which complexes preferentially withuranium and plutonium, which are in solution in the or-ganic phase. The plutonium is separated from the uraniumby a nitric acid solution containing ferrous sulfonate andis removed in the aqueous phase. Further purification ofthe plutonium and uranium streams is required. The ni-tric acid-fission product stream is evaporated for recyclingconcentration and storage in tanks.

STORAGE IN TANKS

All high-activity liquid radioactive wastes are now storedin tanks below the surface. At older U.S. sites, such asHanford, Savannah River, and the NFS fuel reprocessingplant, wastes were stored as alkaline solutions in carbonsteel tanks with a capacity of more than 100,000,000 galin over 200 tanks. Cooling is provided by coils or by con-densation of boiling waste vapors. A schematic of a newertank for high-activity storage is shown in Figure 8.7.3.

Storage in the acid form is now the preferred methodbecause of smaller volumes, no history of leaks, and lessdifficulty with precipitated solids. Current U.S. regulationsallow a 5-yr storage of liquid high-activity wastes in tanksat reprocessing sites before solidifying and shipping to agovernment repository.

Some of the older tanks at the Hanford site were usedto store salt cake remaining after cesium and strontium areremoved from high-activity wastes. In these units, a 3000cfm of 1200°F airflow to an airlift circulator, a 4000 kwelectric immersion heater, and a conventional steam heatedtube bundle evaporator of 6 million btu/hr capacity evap-orates the water and causes the remaining salts to crystal-lize (IAEA 1967). The process is shown schematically forthe electric immersion heater in Figure 8.7.4.

CONVERSION TO SOLIDS

At the Idaho Chemical Processing plant, high-activitywastes were converted to calcined solids in a fluidized bedprocess (Figure 8.7.5). About 250,000 cu ft of aluminaand zirconium wastes containing about 5 3 107 curieshave been solidified between 1963 and 1970. The calcinepowder is blown to stainless steel storage bins. Because thetemperature previously had been limited to 400°C, the re-sultant solid was moderately leachable, and it was fearedthat some fission products, nitrates, and mercuric oxidemight volatilize. Subsequent studies showed that they wereentrapped in the powder. The solids were incorporated


FIG. 8.7.3 Storage of high-activity nuclear wastes in tanks.FIG. 8.7.4 In-tank conversion of radioactive wastes to saltcake.

into a glass matrix or a pot glass system to reduce leach-ability and mobility.

Work on the solidification of high-activity wastes athigh temperatures to obtain a less leachable solid culmi-nated in a demonstration at the Waste SolidificationEngineering Prototype facility at Hanford (Parker 1969).At this plant the batch pot solidification scheme of the OakRidge National Laboratory, the phosphate glass solidifi-cation process of Brookhaven National Laboratory andthe radiant heat spray solidification process of BattelleNorthwest Laboratory were demonstrated at full-scale(light water reactor wastes from fuels irradiated at 45,000mw days/ton at a power level of 30 mw/ton and liquidmetal fast breeder reactor wastes for fuels irradiated at100,000 mw days/ton at 200 mw/ton).

In the pot calcination process, waste feed is batchfed toa heated process vehicle which also serves as the final dis-posal vessel. After the pot is filled, heating continues un-til the waste is converted into a calcine at about 900°C.The pot operates at a constant liquid level and the calcineis deposited radially in the pot. The rate of feed decreasesas the calcined material thickens. The pot is kept at 900°Cuntil all gases are expelled; up to 95% of the pot can befilled.

In the rising-level pot glass process, the necessary addi-tives (H3PO4, NaOH, LiOH, H2O and Al [NO3]3 z 9H2O)to the waste composition are mixed in the feedline or di-rectly in the pot. The process goes through three phases:molten, calcining, and aqueous. The feed into the pot iskept at a low level until the calcine forms and melts. Thefeedrate is then adjusted so that the rate of melting calcineequals the rate at which fresh calcine forms. Therefore, thethree phases are in contact—a rising pool of melt coveredby a thin calcine layer and topped with aqueous waste.Pot calcination has the advantages of (1) simplicity of op-eration; (2) ability to use a wide variety of feeds; (3) re-duction of nitrate content to low levels; (4) minimum ofgas but not constant production; and (5) use of process

vessel as disposal vessel. Its disadvantages include (1) batchprocess; (2) poor heat conduction as calcine builds up onthe walls; and (3) hazard of high organic matter concen-tration.

In the spray calcination process, liquid waste is atom-ized by spraying with steam or air through nozzles at thetop of a stainless steel column, with column walls kept be-tween 600° and 800°C by three-zone heating. The sus-pension of droplets falling down the column dry and arecalcined into a powder. The powder falls into the melter,and process gases and some finer waste powders are blowninto the filter chamber. The powder collects on a porousmetal filter and is occasionally blown off by high pressuresteam, falling into platinum melters where borosilicateglass balls are added to make the melt. The melter oper-ates between 700° and 1300°C, and the molten wasteflows over a weir into the disposal vessel. The advantagesof the spray calciner are: (1) short residence time in thecalciner (safer with thermodynamic, unstable feeds); (2)minimum volume at constant flow of off gases; and (3)utility for wide range of feed composition.

In the continuous phosphate glass solidification process,liquid waste is mixed with phosphoric acid and water, andnitric acid is volatilized at 130°–160°C for a volume re-duction of about 10, and a nitrate removal of about 90%.The solution is then fed to a platinum crucible held at1100°–1200°C, and the melt is then poured into a disposalvessel. The process is continuous and all liquid.

The studies were successfully completed with the pro-cessing of more than 53 million curies in 33 runs, result-ing in solids with a thermal output of 193 kw (Blasewitz1971).

STORAGE

At present, with the exception of hydrofracture and thewaste calcine solids at the National Reactor TestingStation, all high-activity wastes is stored in tanks. Owing


FIG. 8.7.5 Conversion of high-activity waste to calcine solid.

to federal regulations, all high-activity wastes must be con-verted to solids and stored in a federal repository (orderissued on November 14, 1970). The most likely sites forstorage appear to be geologic formations, particularly bed-ded salt.

Geological formations are favored because it is believedthat if their integrity can be maintained, wastes will re-main in place for geologic periods of time. Field scaledemonstrations of 5,000,000 curies of stored fission prod-ucts in irradiated fuel elements at a depth of 1000 ft in asalt mine at Lyons, Kansas, indicate that salt storage, ifthe integrity of the geologic formation can be maintained,will be successful (Bradshaw and McClain 1971). Salt wasfavored as the geologic material because even thoughhighly soluble, it is self-sealing at moderate increases intemperature and pressure, and salt formations are widelyavailable in the United States. Other geologic materialssuch as basalt, gneiss, and schists are also being consid-ered for storage of solidified wastes. The possibility ofhigh-activity liquid waste storage in caverns excavated inbasement rock below the Savannah River plant is beingvigorously pursued.

However, because of uncertainties about long-term(.1000 years) geologic behavior and the effects of storedwastes, more serious consideration is being given to short-term (,100 years) storage of solidified wastes in man-made structures for easier control, maintenance, and re-trieval, if necessary.

—F.L. Parker

ReferencesBlanco, R.E., et al. 1966. Recent developments in treating low and in-

termediate level radioactive wastes in the United States of America.

In Practices in the treatment of low and intermediate level radioac-tive wastes. International Atomic Energy Agency. Vienna.

Blasewitz, A.G. (ed.). 1971. Research and development activities fixationof radioactive residues. (BNWL-1557). Battelle NorthwestLaboratory. Richland, Wash. (February).

Bradshaw, R.L., and W.C. McClain, (eds.). 1971. Project salt vault: Ademonstration of the disposal of high activity solidified wastes in un-derground salt mines (ORNL-4555). Oak Ridge NationalLaboratory. Oak Ridge, Tenn. (April).

De Laguna, W. 1968. Engineering development of hydraulic fracturingas a method for permanent disposal of radioactive wastes (ORNL-4259). Oak Ridge National Laboratory. Oak Ridge, Tenn. (August).

Goldman, M.I. 1968. United States practice in management of radioac-tive wastes at nuclear power plants. Management of radioactivewastes at nuclear power plants. International Atomic Energy Agency.Vienna.

Harvey, R.W., and W.C. Schmidt. 1971. Radioactive waste managementat Hanford. Atlantic Richfield Hanford Company. Richland, Wash.(March).

International Atomic Energy Agency (IAEA). 1967. Disposal of ra-dioactive wastes into the ground. International Atomic EnergyAgency. Vienna.

International Atomic Energy Agency (IAEA). 1970b. Management of lowand intermediate level radioactive wastes. International AtomicEnergy Agency. Vienna.

International Atomic Energy Agency (IAEA). 1970a. Standardization ofradioactive waste categories. International Atomic Energy Agency.Vienna.

Lohse, G.E., D.W. Rhodes, and B.R. Wheeler. 1970. Preventing activityrelease at the Idaho Chemical Processing Plant. In Management oflow and intermediate level radioactive wastes. International AtomicEnergy Agency. Vienna.

Parker, F.L. 1969. Status of radioactive waste disposal in U.S.A. J. Sanit.Engineer. Div., American Society of Civil Engineers 95: SA3. (June).

Straub, C.P. 1964. Low level radioactive wastes. U.S. Atomic EnergyCommission.

United States of America Standards Institute (ASI). 1967. Proposed def-initions of radioactive waste categories. American Institute ofChemical Engineers. New York, N.Y.


chapter 8. removing specific water contaminant

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